MAINTAINERS: document new "K:" entry type

K: is for keyword.  Syntax is perl extended regex.

Reorganized header documentation and indent the section entry descriptions
so that the first K: would not be considered a regex to match by
get_maintainer.pl

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>