ESQL: mention MV_EXPAND on multivalued fields page (#142720)

* ESQL: mention MV_EXPAND on multivalued fields page

Add paragraph explaining that to filter on individual values in a
multivalued field use MV_EXPAND then WHERE. Fixes #105357.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Update docs/reference/query-languages/esql/esql-multivalued-fields.md

Co-authored-by: Liam Thompson <leemthompo@gmail.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Liam Thompson <leemthompo@gmail.com>

Author: 3 people

docs/reference/query-languages/esql/esql-multivalued-fields.md

@@ -264,6 +264,8 @@ Work around this limitation by converting the field to single value with one of:
 * [`MV_MIN`](/reference/query-languages/esql/functions-operators/mv-functions.md#esql-mv_min)
 * [`MV_SUM`](/reference/query-languages/esql/functions-operators/mv-functions.md#esql-mv_sum)
 
+To filter on individual values in a multivalued field (for example, to keep only rows where the field contains a given value), use the [`MV_EXPAND`](/reference/query-languages/esql/commands/mv_expand.md) command to expand the field into one row per value, then apply `WHERE` to the expanded column.
+
 ```console
 POST /_query
 {