Documentation: describe check-ref-format --branch

Unless one already knew, it was not obvious what sort of shorthand
"git check-ref-format --branch" expands.  Explain it.

The --branch argument is not optional.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git check-ref-format' <refname>
-'git check-ref-format' [--branch] <branchname-shorthand>
+'git check-ref-format' --branch <branchname-shorthand>
 
 DESCRIPTION
 -----------
@@ -57,8 +57,11 @@ reference name expressions (see linkgit:git-rev-parse[1]):
 
 . at-open-brace `@{` is used as a notation to access a reflog entry.
 
-With the `--branch` option, it expands a branch name shorthand and
-prints the name of the branch the shorthand refers to.
+With the `--branch` option, it expands the ``previous branch syntax''
+`@{-n}`.  For example, `@{-1}` is a way to refer the last branch you
+were on.  This option should be used by porcelains to accept this
+syntax anywhere a branch name is expected, so they can act as if you
+typed the branch name.
 
 EXAMPLE
 -------