Mention version 1.5.1 in tutorial and user-manual

Most other documentation will frequently be read from an installation
of git so will naturally be associated with the installed version.
But these two documents in particular are often read from web pages
while users are still exploring git. It's important to mention
version 1.5.1 since these documents provide example commands that
won't work with previous versions of git.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/tutorial.txt

-A tutorial introduction to git
-==============================
+A tutorial introduction to git (for version 1.5.1 or newer)
+===========================================================
 
 This tutorial explains how to import a new project into git, make
 changes to it, and share changes with other developers.

Documentation/user-manual.txt

-Git User's Manual
-_________________
+Git User's Manual (for version 1.5.1 or newer)
+______________________________________________
 
 This manual is designed to be readable by someone with basic unix
 command-line skills, but no previous knowledge of git.