[SPARK-36597][DOCS][3.2] Fix issues in SQL function docs

### What changes were proposed in this pull request?

* the functions make_dt_interval and make_ym_interval should make it clear that some of the fields are optional
* remove the `|` symbol from the doc of `bit_get` https://dist.apache.org/repos/dist/dev/spark/v3.2.0-rc1-docs/_site/api/sql/index.html#bit_get
* Address one missing comment in https://github.com/apache/spark/pull/33824#discussion_r695405699

### Why are the changes needed?

Improve the documentation.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Build doc and preview:
![image](https://user-images.githubusercontent.com/1097932/130996918-8c1fff88-ef5a-434b-8445-df7140bad3ba.png)
![image](https://user-images.githubusercontent.com/1097932/130996954-0ced28e7-fb90-4fcc-857e-6ccc31dc3c09.png)

![image](https://user-images.githubusercontent.com/1097932/130955106-5ae32dfc-6e89-4e28-bb8a-6c1b5213051c.png)

![image](https://user-images.githubusercontent.com/1097932/130922351-2f0f262d-5624-4d08-ba83-dfa3ed0b646b.png)

Closes #33857 from gengliangwang/SPARK-36597-3.2.

Authored-by: Gengliang Wang <gengliang@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

project/SparkBuild.scala

@@ -967,6 +967,7 @@ object Unidoc {
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/internal")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/hive")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/catalog/v2/utils")))
+      .map(_.filterNot(_.getCanonicalPath.contains("org.apache.spark.errors")))
       .map(_.filterNot(_.getCanonicalPath.contains("org.apache.spark.sql.errors")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/hive")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/v2/avro")))

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/bitwiseExpressions.scala

@@ -233,10 +233,10 @@ object BitwiseGetUtil {
 
 @ExpressionDescription(
   usage = """
-            |_FUNC_(expr, pos) - Returns the value of the bit (0 or 1) at the specified position.
+    _FUNC_(expr, pos) - Returns the value of the bit (0 or 1) at the specified position.
-            |  The positions are numbered from right to left, starting at zero.
+      The positions are numbered from right to left, starting at zero.
-            |  The position argument cannot be negative.
+      The position argument cannot be negative.
-          """,
+  """,
   examples = """
     Examples:
       > SELECT _FUNC_(11, 0);

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/intervalExpressions.scala

@@ -227,7 +227,7 @@ case class DivideInterval(
 
 // scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(years, months, weeks, days, hours, mins, secs) - Make interval from years, months, weeks, days, hours, mins and secs.",
+  usage = "_FUNC_([years[, months[, weeks[, days[, hours[, mins[, secs]]]]]]]) - Make interval from years, months, weeks, days, hours, mins and secs.",
   arguments = """
     Arguments:
       * years - the number of years, positive or negative
@@ -360,7 +360,7 @@ case class MakeInterval(
 
 // scalastyle:off line.size.limit
 @ExpressionDescription(
-  usage = "_FUNC_(days, hours, mins, secs) - Make DayTimeIntervalType duration from days, hours, mins and secs.",
+  usage = "_FUNC_([days[, hours[, mins[, secs]]]]) - Make DayTimeIntervalType duration from days, hours, mins and secs.",
   arguments = """
     Arguments:
       * days - the number of days, positive or negative
@@ -372,6 +372,8 @@ case class MakeInterval(
     Examples:
       > SELECT _FUNC_(1, 12, 30, 01.001001);
        1 12:30:01.001001000
+      > SELECT _FUNC_(2);
+       2 00:00:00.000000000
       > SELECT _FUNC_(100, null, 3);
        NULL
   """,
@@ -436,7 +438,7 @@ case class MakeDTInterval(
 }
 
 @ExpressionDescription(
-  usage = "_FUNC_(years, months) - Make year-month interval from years, months.",
+  usage = "_FUNC_([years[, months]]) - Make year-month interval from years, months.",
   arguments = """
     Arguments:
       * years - the number of years, positive or negative
@@ -450,6 +452,8 @@ case class MakeDTInterval(
        1-0
       > SELECT _FUNC_(-1, 1);
        -0-11
+      > SELECT _FUNC_(2);
+       2-0
   """,
   since = "3.2.0",
   group = "datetime_funcs")