Update migrate_8_18.asciidoc

Author: georgewallace

docs/reference/migration/migrate_8_18.asciidoc

@@ -90,10 +90,10 @@ EQL queries that would previously fail due to shard failures, will now succeed a
 [%collapsible]
 ====
 *Details* +
-Before this change, ES|QL would honor skip_unavailable setting for nonmatching indices errors at planning time.
+Before this change, ES|QL would honor the skip_unavailable setting for nonmatching indices errors at planning time, meaning that when a query with a concrete (non-wildcarded) index was issued to a cluster where skip_unavailable=true, the index would be skipped, allowing the query to proceed on any other specified clusters rather than returning an error.
 
 *Impact* +
-Remote clusters, regardless of their skip_unavailable setting, will now use the same logic as the local cluster for index expression analysis at plan time
+Remote clusters, regardless of their skip_unavailable setting, will now use the same logic as the local cluster for index expression analysis at plan time. As a result, any missing concrete index in your index expression will return an error rather than partial results.
 ====
 
 [discrete]