documentation: update kernel-doc-nano-HOWTO information

Remove comments about function short descriptions not allowed to be on
multiple lines (that was fixed/changed recently).

Add comments that function "section header:" names need to be unique per
function/struct/union/typedef/enum.

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