docs/pretty-formats: add %(count) and %(total)

When --commit-list-format was introduced to format-patch, two new
placeholders were added to the PRETTY FORMATS code without being
documented. Do so now.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.adoc

@@ -253,6 +253,10 @@ The placeholders are:
 	linkgit:git-rev-list[1])
 +%d+:: ref names, like the --decorate option of linkgit:git-log[1]
 +%D+:: ref names without the " (", ")" wrapping.
++%(count)+:: the number of a patch within a patch series. Used only in
+	`--commit-list-format` in `format-patch`
++%(total)+:: the total number of patches in a patch series. Used only in
+	`--commit-list-format` in `format-patch`
 ++%(decorate++`[:<option>,...]`++)++::
 ref names with custom decorations. The `decorate` string may be followed by a
 colon and zero or more comma-separated options. Option values may contain