Document git-patch-id a bit better.

Pavel Roskin wondered what the SHA1 output at the beginning of
git-diff-tree was about.  The only consumer of that information
so far is this git-patch-id command, which was inadequately
documented.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-patch-id.txt

@@ -18,6 +18,12 @@ ID" are almost guaranteed to be the same thing.
 
 IOW, you can use this thing to look for likely duplicate commits.
 
+When dealing with git-diff-tree output, it takes advantage of
+the fact that the patch is prefixed with the object name of the
+commit, and outputs two 40-byte hexadecimal string.  The first
+string is the patch ID, and the second string is the commit ID.
+This can be used to make a mapping from patch ID to commit ID.
+
 OPTIONS
 -------
 <patch>::