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

Authored-by: srielau <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>
(cherry picked from commit 7786d0b)
Signed-off-by: Wenchen Fan <[email protected]>

Author: srielau

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)