doc/pretty-formats: describe index/time formats for %gd

The "reflog selector" format changes based on a series of
heuristics, and that applies equally to both stock "log -g"
output, as well as "--format=%gd". The documentation for
"%gd" doesn't cover this. Let's mention the multiple formats
and refer the user back to the "-g" section for the complete
rules.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

Documentation/pretty-formats.txt

@@ -147,8 +147,11 @@ endif::git-rev-list[]
   "U" for a good signature with unknown validity and "N" for no signature
 - '%GS': show the name of the signer for a signed commit
 - '%GK': show the key used to sign a signed commit
-- '%gD': reflog selector, e.g., `refs/stash@{1}`
-- '%gd': shortened reflog selector, e.g., `stash@{1}`
+- '%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}`
 - '%gn': reflog identity name
 - '%gN': reflog identity name (respecting .mailmap, see
   linkgit:git-shortlog[1] or linkgit:git-blame[1])