printf(9): clarify the description of %b

The bit values are numbers given in octal representation, not decimal,
as one might assume from the description. Same goes for the base,
although this has an example.

Reviewed by: emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39815

(cherry picked from commit 82bc33d5ad2e00179ea7026c496a35b00550da15)