Say when --track is useful in the git-checkout docs.
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: Federico Mena Quintero <4999915a961edfd7686112c2935288e1266eae14@gnu.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git