[SPARK-31502][SQL][DOCS] Document identifier in SQL Reference

### What changes were proposed in this pull request?
Document identifier in SQL Reference

### Why are the changes needed?
make SQL Reference complete

### Does this PR introduce any user-facing change?
Yes
<img width="1049" alt="Screen Shot 2020-04-23 at 11 14 10 PM" src="https://user-images.githubusercontent.com/13592258/80180695-2f2a4f00-85b8-11ea-819b-f96872956d05.png">

<img width="1050" alt="Screen Shot 2020-04-23 at 11 32 32 PM" src="https://user-images.githubusercontent.com/13592258/80182062-e6c06080-85ba-11ea-9502-1c38358c97c9.png">

### How was this patch tested?
Manually build and check

Closes #28277 from huaxingao/identifier.

Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

docs/_data/menu-sql.yaml

@@ -78,6 +78,8 @@
   subitems:
     - text: Data Types
       url: sql-ref-datatypes.html
+    - text: Identifiers
+      url: sql-ref-identifier.html
     - text: Literals
       url: sql-ref-literals.html
     - text: Null Semantics

docs/sql-ref-identifier.md

@@ -0,0 +1,80 @@
+---
+layout: global
+title: Identifiers
+displayTitle: Identifiers
+license: |
+  Licensed to the Apache Software Foundation (ASF) under one or more
+  contributor license agreements.  See the NOTICE file distributed with
+  this work for additional information regarding copyright ownership.
+  The ASF licenses this file to You under the Apache License, Version 2.0
+  (the "License"); you may not use this file except in compliance with
+  the License.  You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+---
+
+### Description
+
+An identifier is a string used to identify a database object such as a table, view, schema, column, etc. Spark SQL has regular identifiers and delimited identifiers, which are enclosed within backticks. Both regular identifiers and delimited identifiers are case-insensitive.
+
+### Syntax
+
+#### Regular Identifier
+
+{% highlight sql %}
+{ letter | digit | '_' } [ , ... ]
+{% endhighlight %}
+Note: If `spark.sql.ansi.enabled` is set to true, ANSI SQL reserved keywords cannot be used as identifiers. For more details, please refer to [ANSI Compliance](sql-ref-ansi-compliance.html).
+
+#### Delimited Identifier
+
+{% highlight sql %}
+`c [ ... ]`
+{% endhighlight %}
+
+### Parameters
+
+<dl>
+  <dt><code><em>letter</em></code></dt>
+  <dd>
+    Any letter from A-Z or a-z.
+  </dd>
+</dl>
+<dl>
+  <dt><code><em>digit</em></code></dt>
+  <dd>
+    Any numeral from 0 to 9.
+  </dd>
+</dl>
+<dl>
+  <dt><code><em>c</em></code></dt>
+  <dd>
+    Any character from the character set. Use <code>`</code> to escape special characters (e.g., <code>`</code>).
+  </dd>
+</dl>
+
+### Examples
+
+{% highlight sql %}
+-- This CREATE TABLE fails with ParseException because of the illegal identifier name a.b
+CREATE TABLE test (a.b int);
+org.apache.spark.sql.catalyst.parser.ParseException:
+no viable alternative at input 'CREATE TABLE test (a.'(line 1, pos 20)
+
+-- This CREATE TABLE works
+CREATE TABLE test (`a.b` int);
+
+-- This CREATE TABLE fails with ParseException because special character ` is not escaped
+CREATE TABLE test1 (`a`b` int);
+org.apache.spark.sql.catalyst.parser.ParseException:
+no viable alternative at input 'CREATE TABLE test (`a`b`'(line 1, pos 23)
+
+-- This CREATE TABLE works
+CREATE TABLE test (`a``b` int);
+{% endhighlight %}