Documentation: Update information about <format> in git-for-each-ref

Update information about value of <format> used when it is left
unspecified.  Add information about `%%` and `%xx` interpolation
(URL encoding).
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-for-each-ref.txt

@@ -38,7 +38,11 @@ OPTIONS
 	is prefixed with an asterisk (`*`) and the ref points
 	at a tag object, the value for the field in the object
 	tag refers is used.  When unspecified, defaults to
-	`%(refname)`.
+	`%(objectname) SPC %(objecttype) TAB %(refname)`.
+	It also interpolates `%%` to `%`, and `%xx` where `xx`
+	are hex digits interpolates to character with hex code
+	`xx`; for example `%00` interpolates to `\0` (NUL),
+	`%09` to `\t` (TAB) and `%0a` to `\n` (LF).
 
 <pattern>::
 	If given, the name of the ref is matched against this