doc/pretty-formats: explain shortening of %gd

The actual shortening rules aren't that interesting and
probably not worth getting into (I gloss over them here as
"shortened for human readability"). But the fact that %gD
shows whatever you gave on the command line is subtle and
worth mentioning. Since most people will feed a shortened
refname in the first place, it otherwise makes it hard to
understand the difference between the two.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -149,9 +149,12 @@ endif::git-rev-list[]
 - '%GK': show the key used to sign a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}` or
   `refs/stash@{2 minutes ago`}; the format follows the rules described
-  for the `-g` option
-- '%gd': shortened reflog selector, e.g., `stash@{1}` or
-  `stash@{2 minutes ago}`
+  for the `-g` option. The portion before the `@` is the refname as
+  given on the command line (so `git log -g refs/heads/master` would
+  yield `refs/heads/master@{0}`).
+- '%gd': shortened reflog selector; same as `%gD`, but the refname
+  portion is shortened for human readability (so `refs/heads/master`
+  becomes just `master`).
 - '%gn': reflog identity name
 - '%gN': reflog identity name (respecting .mailmap, see
   linkgit:git-shortlog[1] or linkgit:git-blame[1])