clock_gettime.2: add some clock IDs

Add the CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clock_id
values to the clock_gettime(2) man page. Reformat the excessively
long paragraph (sentence!) into a tag list.

Reported by: jilles in https://reviews.freebsd.org/D10020
MFC after: 3 days
Sponsored by: Dell EMC