format-patch: give an overview of what a "patch" message is

The text says something called a "patch" is prepared one for each
commit, it is suitable for e-mail submission, and "am" is the
command to use it, but does not say what the "patch" really is.

The description in the page also refers to the "three-dash" line,
but it is unclear what it is, unless the reader is given a more
detailed overview of what the "patch" is.

Add a brief paragraph to give an overview of what the output looks
like.

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

Documentation/git-format-patch.txt

@@ -36,11 +36,28 @@ SYNOPSIS
 DESCRIPTION
 -----------
 
-Prepare each commit with its patch in
+Prepare each commit with its "patch" in
-one file per commit, formatted to resemble UNIX mailbox format.
+one "message" per commit, formatted to resemble a UNIX mailbox.
 The output of this command is convenient for e-mail submission or
 for use with 'git am'.
 
+A "message" generated by the command consists of three parts:
+
+* A brief metadata header that begins with `From <commit>`
+  with a fixed `Mon Sep 17 00:00:00 2001` datestamp to help programs
+  like "file(1)" to recognize that the file is an output from this
+  command, fields that record the author identity, the author date,
+  and the title of the change (taken from the first paragraph of the
+  commit log message).
+
+* The second and subsequent paragraphs of the commit log message.
+
+* The "patch", which is the "diff -p --stat" output (see
+  linkgit:git-diff[1]) between the commit and its parent.
+
+The log message and the patch is separated by a line with a
+three-dash line.
+
 There are two ways to specify which commits to operate on.
 
 1. A single commit, <since>, specifies that the commits leading