Clarify the documentation of pmcstat:
the -d argument should be passed before -p, -s, -P or -S to be taken in account

Differential Revision:	https://reviews.freebsd.org/D1011
Reviewed by:	adrian, gnn
MFC after:	1 week
Correct a typo: this is the manpage for pthread_cleanup_pop, not push.

Submitted by:	ian
Fix rendering

Submitted by:	brueffer