The rest of the man page also uses the imperative form for subcommands, so it might be nice to keep it and use that instead. Also the wording looks a bit strange. I'd suggest reusing the cmd_help as the first phrase and expanding on that, just an idea:
```
Checkout a remote branch taken from a pull request. This command first
fetches the *head* reference from the pull request and then calls the
standard `git checkout` command and any extra argument will be passed
to `git checkout` as-is, after the reference that was just fetched.
Remember this creates a detached checkout by default, use `-b` if you
want to create a new branch based on the pull request. Please take a
look at `git checkout` help for more details.
```