Docs: Renamed Threads to Comments and threads

Related to: https://gitlab.com/gitlab-org/gitlab-docs/-/merge_requests/1899

doc/user/discussions/index.md

@@ -5,7 +5,7 @@ info: "To determine the technical writer assigned to the Stage/Group associated
 type: reference, howto
 ---
 
-# Threads **(FREE)**
+# Comments and threads **(FREE)**
 
 GitLab encourages communication through comments, threads, and
 [code suggestions](../project/merge_requests/reviews/suggestions.md).