Documentation: clarify %(contents:XXXX) doc

Let's avoid a big dense paragraph by using an unordered
list for the %(contents:XXXX) format specifiers.

While at it let's also make the following improvements:

  - Let's not describe %(contents) using "complete message"
    as it's not clear what an incomplete message is.

  - Let's improve how the "subject" and "body" are
    described.

  - Let's state that "signature" is only available for
    tag objects.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -232,12 +232,24 @@ Fields that have name-email-date tuple as its value (`author`,
 `committer`, and `tagger`) can be suffixed with `name`, `email`,
 and `date` to extract the named component.
 
-The complete message in a commit and tag object is `contents`.
-Its first line is `contents:subject`, where subject is the concatenation
-of all lines of the commit message up to the first blank line.  The next
-line is `contents:body`, where body is all of the lines after the first
-blank line.  The optional GPG signature is `contents:signature`.  The
-first `N` lines of the message is obtained using `contents:lines=N`.
+The message in a commit or a tag object is `contents`, from which
+`contents:<part>` can be used to extract various parts out of:
+
+contents:subject::
+	The first paragraph of the message, which typically is a
+	single line, is taken as the "subject" of the commit or the
+	tag message.
+
+contents:body::
+	The remainder of the commit or the tag message that follows
+	the "subject".
+
+contents:signature::
+	The optional GPG signature of the tag.
+
+contents:lines=N::
+	The first `N` lines of the message.
+
 Additionally, the trailers as interpreted by linkgit:git-interpret-trailers[1]
 are obtained as `trailers` (or by using the historical alias
 `contents:trailers`).  Non-trailer lines from the trailer block can be omitted