diff-tree: Note that the commit ID is printed with --stdin

It's sort of already documented with the --no-commit-id command-line
flag, but let's not hide important information from the user.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff-tree.txt

@@ -52,10 +52,14 @@ include::diff-options.txt[]
 	reads either one <commit> or a list of <commit>
 	separated with a single space from its standard input.
 +
-When a single commit is given on one line of such input, it compares
-the commit with its parents.  The following flags further affects its
-behavior.  The remaining commits, when given, are used as if they are
+When a single commit is given, it compares the commit with its
+parents.  The remaining commits, when given, are used as if they are
 parents of the first commit.
++
+The ID of the first (or only) commit, followed by a newline, is
+printed before the differences.
++
+The following flags further affects its behavior.
 
 -m::
 	By default, 'git-diff-tree --stdin' does not show