git-filter-repo.txt: link to GitHub docs on purging old history

Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/git-filter-repo.txt

@@ -563,7 +563,11 @@ history rewrite are roughly as follows:
      hosting provider about how to remove any server-side references
      to the old commits (example:
      https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html[GitLab's
-     docs on reducing repository size]).
+     excellent docs on reducing repository size], or just the warning
+     box that references "GitHub support" from
+     https://docs.github.com/en/github/authenticating-to-github/removing-sensitive-data-from-a-repository[GitHub's
+     otherwise dangerously out-of-date docs on removing sensitive
+     data]).
 
 6. (Optional) Some additional considerations