Add that description diffs are behind flag

Updates documentation to state that description diffs are
behind a feature flag.

doc/user/project/issues/issue_data_and_actions.md

@@ -157,7 +157,7 @@ The plain text title and description of the issue fill the top center of the iss
 The description fully supports [GitLab Flavored Markdown](../../markdown.md#gitlab-flavored-markdown-gfm),
 allowing many formatting options.
 
-> [Since GitLab 12.5](https://gitlab.com/gitlab-org/gitlab/issues/10103), changes to an issue's description are listed in the [issue history](#23-issue-history).**(STARTER)**
+> [Since GitLab 12.6](https://gitlab.com/gitlab-org/gitlab/issues/10103), changes to an issue's description are listed in the [issue history](#23-issue-history).**(STARTER)**
 
 #### 17. Mentions