bitset.9: add description of BIT_ISSET

Extracted from abandoned review https://reviews.freebsd.org/D33701.

(cherry picked from commit 145c65fc5791d1e4a21c7653f68a55cfd1772aac)

cpuset.9: add description of CPU_ISSET

(cherry picked from commit eaf3c3af9c88dc63ea09833397f2bbeded16f553)