remove shell pseudocode for branch rename

The rationale here is that's already available at the head of the
Python script, and was actually not directly tested recently. I only
put this together from memory (well, git history and guesses) so it
might actually not work at all. It's mostly pseudo-code, and shouldn't
be ran directly.

Instead, explain generally, with words, what the script does.

howto/gitlab.md

@@ -469,31 +469,9 @@ branch. That process is more involved.
 
 Changing the name of a default branch both locally and on remotes can
 be partially automated with the use of [anarcat's branch rename
-script](https://gitlab.com/anarcat/scripts/-/blob/main/git-branch-rename-remote). This script performs the following tasks:
-
-```
-# rename the branch locally if not already done
-git show-ref refs/heads/master > /dev/null && \
-  git branch --move master main
-git fetch origin
-# fix HEAD pointer for local remote branch
-git symbolic-ref refs/remotes/remote/HEAD refs/remotes/remote/main
-# push branch remotely if it doesn't exist (or fix tracking branch if it does)
-git branch -u origin/main || exit
-git push -u origin main
-# fix remote HEAD pointer on remote repository
-ssh example.com git symbolic-ref HEAD main
-# delete old branch
-git push -d origin master
-```
-
-If the `ssh` step fails, it will try to do the equivalent over the
-GitLab API, that is:
-
- 1. unprotect the old branch
- 2. protect the new branch
- 3. change the default branch
- 4. *then* `git push -d` to delete the old branch
+script](https://gitlab.com/anarcat/scripts/-/blob/main/git-branch-rename-remote). The script basically renames the branch locally, pushes
+the new branch and deletes the old one, with special handling of
+GitLab remotes, where it "un-protects" and "re-protects" the branch.
 
 You should run the script with an account that has "Maintainer" or
 "Owner" access to GitLab, so that it can do the above GitLab API