pcm.4: Fix lint warnings

Ignore the "manual not found" warnings for snd_ai2s(4) and
snd_davbus(4).

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43996

(cherry picked from commit 8bfb23abf882afdca0a0530ef1231ac59963bc4a)