Clarify that pthread_cleanup_push()/pop() are implemented as macros that
create a new code block and thus must be balanced at the same lexical
scope.  (This is also a requirement in POSIX.)

PR:		194280
Submitted by:	dr2867.business@pacbell.net
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273644 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f