Say when --track is useful in the git-checkout docs.

The documentation used to say what the option does, but it
didn't mention a use case.
Signed-off-by: NFederico Mena Quintero <federico@gnu.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -50,7 +50,9 @@ OPTIONS
 --track::
 	When -b is given and a branch is created off a remote branch,
 	set up configuration so that git-pull will automatically
-	retrieve data from the remote branch.  Set the
+	retrieve data from the remote branch.  Use this if you always
+	pull from the same remote branch into the new branch, or if you
+	don't want to use "git pull <repository> <refspec>" explicitly.  Set the
 	branch.autosetupmerge configuration variable to true if you
 	want git-checkout and git-branch to always behave as if
 	'--track' were given.