check-ref-format doc: --branch validates and expands <branch>

"git check-ref-format --branch $name" feature was originally
introduced (and was advertised) as a way for scripts to take any
end-user supplied string (like "master", "@{-1}" etc.) and see if it
is usable when Git expects to see a branch name, and also obtain the
concrete branch name that the at-mark magic expands to.

Emphasize that "see if it is usable" role in the description and
clarify that the @{...} expansion only occurs when run from within a
repository.

[jn: split out from a larger patch]

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -77,7 +77,14 @@ reference name expressions (see linkgit:gitrevisions[7]):
 
 . at-open-brace `@{` is used as a notation to access a reflog entry.
 
-With the `--branch` option, it expands the ``previous branch syntax''
+With the `--branch` option, the command takes a name and checks if
+it can be used as a valid branch name (e.g. when creating a new
+branch).  The rule `git check-ref-format --branch $name` implements
+may be stricter than what `git check-ref-format refs/heads/$name`
+says (e.g. a dash may appear at the beginning of a ref component,
+but it is explicitly forbidden at the beginning of a branch name).
+When run with `--branch` option in a repository, the input is first
+expanded for 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