Merge branch 'jh-document-and-lock-markdown-cache' into 'master'

Document locking of markdown cache version

See merge request gitlab-org/gitlab!61505

lib/gitlab/markdown_cache.rb

@@ -2,7 +2,11 @@
 
 module Gitlab
   module MarkdownCache
-    # Increment this number every time the renderer changes its output
+    # Increment this number every time the renderer changes its output.
+    # Changing this value puts strain on the database, as every row with
+    # cached markdown needs to be updated. As a result, this line should
+    # not be changed.
+    # See: https://gitlab.com/gitlab-org/gitlab/-/issues/330313
     CACHE_COMMONMARK_VERSION        = 28
     CACHE_COMMONMARK_VERSION_START  = 10