[DOCS][8.x] ESQL: Document LU JOIN/MV_EXPAND not respecting SORT (#127813) (#127816)

* [DOCS][8.x] ESQL: Document LU JOIN/MV_EXPAND not respecting SORT

* Fix nasty asciidoc backticks

docs/reference/esql/esql-lookup-join.asciidoc

@@ -231,6 +231,29 @@ To obtain a join key with a compatible type, use a
 For a complete list of supported data types and their internal
 representations, see the <<esql-supported-types,Supported Field Types documentation>>.
 
+[discrete]
+[[esql-lookup-join-usage-notes]]
+==== Usage notes
+
+This section covers important details about `LOOKUP JOIN` that impact query behavior and results. Review these details to ensure your queries work as expected and to troubleshoot unexpected results.
+
+[discrete]
+[[esql-lookup-join-usage-notes-name-collisions]]
+===== Handling name collisions
+
+When fields from the lookup index match existing column names, the new columns override the existing ones.
+Before the `LOOKUP JOIN` command, preserve columns by either:
+
+* Using `RENAME` to assign non-conflicting names
+* Using `EVAL` to create new columns with different names
+
+[discrete]
+[[esql-lookup-join-usage-notes-sorting]]
+===== Sorting behavior
+
+The output rows produced by `LOOKUP JOIN` can be in any order and may not
+respect preceding `SORT` commands. To guarantee a certain ordering, place a `SORT` after any `LOOKUP JOIN` commands.
+
 [discrete]
 [[esql-lookup-join-limitations]]
 ==== Limitations

docs/reference/esql/processing-commands/lookup.asciidoc

@@ -50,7 +50,7 @@ results, the output will contain one row for each matching combination.
 
 [TIP]
 ====
-In case of name collisions, the newly created columns will override existing columns.
+For important information about using `LOOKUP JOIN`, refer to <<esql-lookup-join-usage-notes,Usage notes>>.
 ====
 
 *Examples*

docs/reference/esql/processing-commands/mv_expand.asciidoc

@@ -19,6 +19,13 @@ MV_EXPAND column
 `column`::
 The multivalued column to expand.
 
+[WARNING]
+======
+The output rows produced by `MV_EXPAND` can be in any order and may not respect
+preceding `SORT` commands. To guarantee a certain ordering, place a `SORT` after any
+`MV_EXPAND` commands.
+======
+
 *Example*
 
 [source.merge.styled,esql]