mktag: minimally update the description.

It lacked a description for the (historically) optional tagger header line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mktag.txt

@@ -19,18 +19,18 @@ The output is the new tag's <object> identifier.
 
 Tag Format
 ----------
-A tag signature file has a very simple fixed format: three lines of
+A tag signature file has a very simple fixed format: four lines of
 
   object <sha1>
   type <typename>
   tag <tagname>
+  tagger <tagger>
 
-followed by some 'optional' free-form signature that git itself
-doesn't care about, but that can be verified with gpg or similar.
-
-The size of the full object is artificially limited to 8kB.  (Just
-because I'm a lazy bastard, and if you can't fit a signature in that
-size, you're doing something wrong)
+followed by some 'optional' free-form message (some tags created
+by older git may not have `tagger` line).  The message, when
+exists, is separated by a blank line from the header.  The
+message part may contain a signature that git itself doesn't
+care about, but that can be verified with gpg.
 
 
 Author