Documentation/applying-patches.txt: Update the information there

This document is old: it is from Kernel v2.6.12 days.
Update it to the current status, and add a reference for the
linux-next tree.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>