ESQL: Updated RENAME docs with the behaviour of multiple column renames (elastic#126462) (elastic#126522)

* ESQL: Updated RENAME docs with the behaviour of multiple column renames

* Added rename example to csv-spec

Author: ivancea

docs/reference/query-languages/esql/esql-commands.md

@@ -802,6 +802,8 @@ RENAME old_name1 AS new_name1[, ..., old_nameN AS new_nameN]
 
 The `RENAME` processing command renames one or more columns. If a column with the new name already exists, it will be replaced by the new column.
 
+A `RENAME` with multiple column renames is equivalent to multiple sequential `RENAME` commands.
+
 **Examples**
 
 ```esql
@@ -818,6 +820,15 @@ FROM employees
 | RENAME first_name AS fn, last_name AS ln
 ```
 
+With multiple `RENAME` commands:
+
+```esql
+FROM employees
+| KEEP first_name, last_name
+| RENAME first_name AS fn
+| RENAME last_name AS ln
+```
+
 
 ## `SORT` [esql-sort]
 

x-pack/plugin/esql/qa/testFixtures/src/main/resources/docs.csv-spec

@@ -112,6 +112,18 @@ FROM employees
 fn:keyword | ln:keyword
 ;
 
+docsRenameMultipleColumnsDifferentCommands
+// tag::renameMultipleColumnsDifferentCommands[]
+FROM employees
+| KEEP first_name, last_name
+| RENAME first_name AS fn
+| RENAME last_name AS ln
+// end::renameMultipleColumnsDifferentCommands[]
+| LIMIT 0;
+
+fn:keyword | ln:keyword
+;
+
 docsSort
 // tag::sort[]
 FROM employees