Documentation: clarify track/no-track option.

Fix the description of the --no-track option so it no longer says the
opposite of what was intended.  Also mention branch.autosetupmerge
explicitly.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-checkout.txt

@@ -49,13 +49,17 @@ OPTIONS
 
 --track::
 	When -b is given and a branch is created off a remote branch,
-	setup so that git-pull will automatically retrieve data from
-	the remote branch.
+	set up configuration so that git-pull will automatically
+	retrieve data from the remote branch.  Set the
+	branch.autosetupmerge configuration variable to true if you
+	want git-checkout and git-branch to always behave as if
+	'--track' were given.
 
 --no-track::
 	When -b is given and a branch is created off a remote branch,
-	force that git-pull will automatically retrieve data from
-	the remote branch independent of the configuration settings.
+	set up configuration so that git-pull will not retrieve data
+	from the remote branch, ignoring the branch.autosetupmerge
+	configuration variable.
 
 -l::
 	Create the new branch's ref log.  This activates recording of