[SPARK-28786][DOC][SQL][FOLLOW-UP] Change "Related Statements" to bold

### What changes were proposed in this pull request?
Change "Related Statements" to bold

### Why are the changes needed?
To make doc look nice and consistent.

### Does this PR introduce any user-facing change?
Yes

### How was this patch tested?
Tested using jykyll build --serve

Before the change:
![image](https://user-images.githubusercontent.com/13592258/63965303-ae797a00-ca4d-11e9-8a85-71fbfdeaaccb.png)

After the change:
![image](https://user-images.githubusercontent.com/13592258/63965316-b76a4b80-ca4d-11e9-9a85-48d7a909f0ef.png)

Before the change:
![image](https://user-images.githubusercontent.com/13592258/63988989-7c8b0680-ca93-11e9-9352-a9ec5457b279.png)

After the change:
![image](https://user-images.githubusercontent.com/13592258/63988996-87459b80-ca93-11e9-9e51-8cb36a632436.png)

Closes #25623 from huaxingao/spark-28786-n.

Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

docs/sql-ref-syntax-dml-insert-into.md

@@ -95,9 +95,8 @@ INSERT INTO [ TABLE ] table_name
 {% endhighlight %}
 
 #### Insert Using a SELECT Statement
-Assuming the `persons` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the persons table has already been created and populated.
  SELECT * FROM persons;
 
      + -------------- + ------------------------------ + -------------- +
@@ -127,9 +126,8 @@ Assuming the `persons` table has already been created and populated.
 {% endhighlight %}
 
 #### Insert Using a TABLE Statement
-Assuming the `visiting_students` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the visiting_students table has already been created and populated.
  SELECT * FROM visiting_students;
 
      + -------------- + ------------------------------ + -------------- +
@@ -162,9 +160,8 @@ Assuming the `visiting_students` table has already been created and populated.
 {% endhighlight %}
 
 #### Insert Using a FROM Statement
-Assuming the `applicants` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the applicants table has already been created and populated.
  SELECT * FROM applicants;
 
      + -------------- + ------------------------------ + -------------- + -------------- +
@@ -203,7 +200,7 @@ Assuming the `applicants` table has already been created and populated.
      + -------------- + ------------------------------ + -------------- +
 {% endhighlight %}
 
-Related Statements:
+### Related Statements
   * [INSERT OVERWRITE statement](sql-ref-syntax-dml-insert-overwrite-table.html)
   * [INSERT OVERWRITE DIRECTORY statement](sql-ref-syntax-dml-insert-overwrite-directory.html)
   * [INSERT OVERWRITE DIRECTORY with Hive format statement](sql-ref-syntax-dml-insert-overwrite-directory-hive.html)

docs/sql-ref-syntax-dml-insert-overwrite-directory-hive.md

@@ -81,7 +81,7 @@ INSERT OVERWRITE [ LOCAL ] DIRECTORY directory_path
      SELECT * FROM test_table;
 {% endhighlight %}
 
-Related Statements:
+### Related Statements
   * [INSERT INTO statement](sql-ref-syntax-dml-insert-into.html)
   * [INSERT OVERWRITE statement](sql-ref-syntax-dml-insert-overwrite-table.html)
   * [INSERT OVERWRITE DIRECTORY statement](sql-ref-syntax-dml-insert-overwrite-directory.html)

docs/sql-ref-syntax-dml-insert-overwrite-directory.md

@@ -79,7 +79,7 @@ INSERT OVERWRITE DIRECTORY
     SELECT * FROM test_table;
 {% endhighlight %}
 
-Related Statements:
+### Related Statements
   * [INSERT INTO statement](sql-ref-syntax-dml-insert-into.html)
   * [INSERT OVERWRITE statement](sql-ref-syntax-dml-insert-overwrite-table.html)
   * [INSERT OVERWRITE DIRECTORY with Hive format statement](sql-ref-syntax-dml-insert-overwrite-directory-hive.html)

docs/sql-ref-syntax-dml-insert-overwrite-table.md

@@ -25,7 +25,7 @@ The `INSERT OVERWRITE` statement overwrites the existing data in the table using
 
 ### Syntax
 {% highlight sql %}
-INSERT OVERWRITE [TABLE] table_name
+INSERT OVERWRITE [ TABLE ] table_name
     [ PARTITION ( partition_col_name [ = partition_col_val ] [ , ... ] ) [ IF NOT EXISTS ] ]
     { { VALUES ( { value | NULL } [ , ... ] ) [ , ( ... ) ] } | query }
 {% endhighlight %}
@@ -59,10 +59,8 @@ INSERT OVERWRITE [TABLE] table_name
 
 ### Examples
 #### Insert Using a VALUES Clause
-
-Assuming the `students` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the students table has already been created and populated.
  SELECT * FROM students;
 
      + -------------- + ------------------------------ + -------------- +
@@ -101,9 +99,8 @@ Assuming the `students` table has already been created and populated.
 {% endhighlight %}
 
 #### Insert Using a SELECT Statement
-Assuming the `persons` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the persons table has already been created and populated.
  SELECT * FROM persons;
 
       + -------------- + ------------------------------ + -------------- +
@@ -129,9 +126,8 @@ Assuming the `persons` table has already been created and populated.
 {% endhighlight %}
 
 #### Insert Using a TABLE Statement
-Assuming the `visiting_students` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the visiting_students table has already been created and populated.
  SELECT * FROM visiting_students;
 
       + -------------- + ------------------------------ + -------------- +
@@ -156,9 +152,8 @@ Assuming the `visiting_students` table has already been created and populated.
 {% endhighlight %}
 
 #### Insert Using a FROM Statement
-Assuming the `applicants` table has already been created and populated.
-
 {% highlight sql %}
+ -- Assuming the applicants table has already been created and populated.
  SELECT * FROM applicants;
 
      + -------------- + ------------------------------ + -------------- + -------------- +
@@ -185,7 +180,7 @@ Assuming the `applicants` table has already been created and populated.
      + -------------- + ------------------------------ + -------------- +
 {% endhighlight %}
 
-Related Statements:
+### Related Statements
   * [INSERT INTO statement](sql-ref-syntax-dml-insert-into.html)
   * [INSERT OVERWRITE DIRECTORY statement](sql-ref-syntax-dml-insert-overwrite-directory.html)
   * [INSERT OVERWRITE DIRECTORY with Hive format statement](sql-ref-syntax-dml-insert-overwrite-directory-hive.html)