Style(9): Strengthen statements about not using K&R function definitions

K&R function definitions will soon be obsolete. Work has been underway
to remove all K&R function definitions from the tree for a while now. A
future C version will remove this construct from the language. So
strengthen existing statements about K&R function definitions and
declarations.

While here, remove __P macro reference. It's not been in active use for
almost two decades apart from legacy contrib code.

Sponsored by: Netflix
Reviewed by: pauamma, rpokala, hselasky, kp, brooks, markm, emaste
Differential Revision: https://reviews.freebsd.org/D35945