documentation: update CodingStyle tips for Emacs users

Describe a setup that integrates better with Emacs' cc-mode and also fixes
up the alignment of continuation lines to really only use tabs.

Signed-off-by: Johannes Weiner <hannes@saeurebad.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>