documentation: how to use DOC: section blocks

Add info on how to use DOC: sections in kernel-doc.  DOC: sections enable
the addition of inline source file comments that are general in nature
instead of being specific to a function, struct, union, enum, or typedef.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>