Documentation/kbuild: major edit of modules.txt sections 5-8

A follow-up to my edit of the first 4 sections.

Shift sections down by one due to the deletion of section 3; grammar
corrections along with some rewording; margin width cleanup; and
change EXTRA_CFLAGS -> ccflags-y.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>