[SPARK-43205][DOC] identifier clause docs

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

Document the IDENTIFIER() clause

### Why are the changes needed?

Docs are good!

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

### How was this patch tested?
<!--
<img width="892" alt="Screenshot 2023-08-15 at 4 26 27 PM" src="https://github.com/apache/spark/assets/3514644/6ce43330-668e-4c84-b72b-bf1e2679d736">

If tests were added, say they were added here. Please make sure to add some test cases that check the changes thoroughly including negative and positive cases if possible.
If it was tested in a way different from regular unit tests, please clarify how you tested step by step, ideally copy and paste-able, so that other reviewers can test and check, and descendants can verify in the future.
If tests were not added, please describe why they were not added and/or why it was difficult to add.
If benchmark tests were added, please run the benchmarks in GitHub Actions for the consistent environment, and the instructions could accord to: https://spark.apache.org/developer-tools.html#github-workflow-benchmarks.
-->
See attached
<img width="892" alt="Screenshot 2023-08-15 at 4 26 27 PM" src="https://github.com/apache/spark/assets/3514644/55823375-8d1a-4473-bf19-74796d273416">

<img width="747" alt="Screenshot 2023-08-15 at 4 45 23 PM" src="https://github.com/apache/spark/assets/3514644/0ee852a9-6a11-4c87-bed9-43531c55fc31">

Closes #42506 from srielau/SPARK-43205-3.5-IDENTIFIER-clause-docs.

Authored-by: srielau <serge@rielau.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

docs/sql-ref-identifier-clause.md

@@ -0,0 +1,106 @@
+---
+layout: global
+title: Identifier clause
+displayTitle: IDENTIFIER clause
+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
+
+Converts a constant `STRING` expression into a SQL object name.
+The purpose of this clause is to allow for templating of identifiers in SQL statements without opening up the risk of SQL injection attacks.
+Typically, this clause is used with a parameter marker as argument.
+
+### Syntax
+
+```sql
+IDENTIFIER ( strExpr )
+```
+
+### Parameters
+
+- **strExpr**: A constant `STRING` expression. Typically, the expression includes a parameter marker.
+
+### Returns
+
+A (qualified) identifier which can be used as a:
+
+- qualified table name
+- namespace name
+- function name
+- qualified column or attribute reference
+
+### Examples
+
+These examples use named parameter markers to templatize queries.
+
+```scala
+// Creation of a table using parameter marker.
+spark.sql("CREATE TABLE IDENTIFIER(:mytab)(c1 INT)", args = Map("mytab" -> "tab1")).show()
+
+spark.sql("DESCRIBE IDENTIFIER(:mytab)", args = Map("mytab" -> "tab1")).show()
++--------+---------+-------+
+|col_name|data_type|comment|
++--------+---------+-------+
+|      c1|      int|   NULL|
++--------+---------+-------+
+
+// Altering a table with a fixed schema and a parameterized table name. 
+spark.sql("ALTER TABLE IDENTIFIER('default.' || :mytab) ADD COLUMN c2 INT", args = Map("mytab" -> "tab1")).show()
+
+spark.sql("DESCRIBE IDENTIFIER(:mytab)", args = Map("mytab" -> "default.tab1")).show()
++--------+---------+-------+
+|col_name|data_type|comment|
++--------+---------+-------+
+|      c1|      int|   NULL|
+|      c2|      int|   NULL|
++--------+---------+-------+
+
+// A parameterized reference to a table in a query. This table name is qualified and uses back-ticks.
+spark.sql("SELECT * FROM IDENTIFIER(:mytab)", args = Map("mytab" -> "`default`.`tab1`")).show()
++---+---+
+| c1| c2|
++---+---+
++---+---+
+
+
+// You cannot qualify the IDENTIFIER clause or use it as a qualifier itself.
+spark.sql("SELECT * FROM myschema.IDENTIFIER(:mytab)", args = Map("mytab" -> "`tab1`")).show()
+[INVALID_SQL_SYNTAX.INVALID_TABLE_VALUED_FUNC_NAME] `myschema`.`IDENTIFIER`.
+
+spark.sql("SELECT * FROM IDENTIFIER(:myschema).mytab", args = Map("mychema" -> "`default`")).show()
+[PARSE_SYNTAX_ERROR]
+
+// Dropping a table with separate schema and table parameters.
+spark.sql("DROP TABLE IDENTIFIER(:myschema || '.' || :mytab)", args = Map("myschema" -> "default", "mytab" -> "tab1")).show()
+
+// A parameterized column reference
+spark.sql("SELECT IDENTIFIER(:col) FROM VALUES(1) AS T(c1)", args = Map("col" -> "t.c1")).show()
++---+
+| c1|
++---+
+|  1|
++---+
+
+// Passing in a function name as a parameter
+spark.sql("SELECT IDENTIFIER(:func)(-1)", args = Map("func" -> "abs")).show();
++-------+
+|abs(-1)|
++-------+
+|      1|
++-------+
+```

docs/sql-ref.md

@@ -32,6 +32,7 @@ Spark SQL is Apache Spark's module for working with structured data. This guide
    * [User-Defined Aggregate Functions (UDAFs)](sql-ref-functions-udf-aggregate.html)
    * [Integration with Hive UDFs/UDAFs/UDTFs](sql-ref-functions-udf-hive.html)
  * [Identifiers](sql-ref-identifier.html)
+ * [IDENTIFIER clause](sql-ref-identifier-clause.html)
  * [Literals](sql-ref-literals.html)
  * [Null Semantics](sql-ref-null-semantics.html)
  * [SQL Syntax](sql-ref-syntax.html)