Merge branch 'docs/cross-link-default-branch' into 'master'

Cross link default branch docs to issue closing pattern docs

Closes #40449

See merge request gitlab-org/gitlab-ce!15546

doc/user/project/issues/automatic_issue_closing.md

@@ -12,8 +12,9 @@ in the project's default branch.
 
 If a commit message or merge request description contains a sentence matching
 a certain regular expression, all issues referenced from the matched text will
-be closed. This happens when the commit is pushed to a project's **default**
-branch, or when a commit or merge request is merged into it.
+be closed. This happens when the commit is pushed to a project's
+[**default** branch](../repository/branches/index.md#default-branch), or when a
+commit or merge request is merged into it.
 
 ## Default closing pattern value