[DOCS] Add text_expansion deprecation usage note (#115529)

(cherry picked from commit 6980fc6)

Author: leemthompo

docs/reference/query-dsl/text-expansion-query.asciidoc

@@ -7,6 +7,13 @@
 
 deprecated[8.15.0, This query has been replaced by <<query-dsl-sparse-vector-query>>.]
 
+.Deprecation usage note
+****
+You can continue using `rank_features` fields with `text_expansion` queries in the current version.
+However, if you plan to upgrade, we recommend updating mappings to use the `sparse_vector` field type and <<docs-reindex,reindexing your data>>.
+This will allow you to take advantage of the new capabilities and improvements available in newer versions.
+****
+
 The text expansion query uses a {nlp} model to convert the query text into a list of token-weight pairs which are then used in a query against a
 <<sparse-vector,sparse vector>> or <<rank-features,rank features>> field.