Add clarification about PRs and renaming branches (github#40025)

Author: isaacmbrown

content/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch.md

@@ -17,7 +17,9 @@ redirect_from:
 
 You can rename a branch in a repository on {% data variables.location.product_location %}. For more information about branches, see "[AUTOTITLE](/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-branches))."
 
-When you rename a branch on {% data variables.location.product_location %}, any URLs that contain the old branch name are automatically redirected to the equivalent URL for the renamed branch. Branch protection policies are also updated, as well as the base branch for open pull requests (including those for forks) and draft releases. After the rename is complete, {% data variables.product.prodname_dotcom %} provides instructions on the repository's home page directing contributors to update their local Git environments.
+When you rename a branch on {% data variables.location.product_location %}, any URLs that contain the old branch name are automatically redirected to the equivalent URL for the renamed branch. Branch protection policies are also updated, as well as the base branch for open pull requests (including those for forks) and draft releases. If the renamed branch is the head branch of an open pull request, this pull request is closed.
+
+After the rename is complete, {% data variables.product.prodname_dotcom %} provides instructions on the repository's home page directing contributors to update their local Git environments.
 
 Although file URLs are automatically redirected, raw file URLs are not redirected. Also, {% data variables.product.prodname_dotcom %} does not perform any redirects if users perform a `git pull` for the previous branch name.