critical(9): small updates

• Document CRITICAL_ASSERT() in this man page.
• Clarify that a thread may also handle interrupts in a critical section, not only faults/exceptions.
• Note the negative effects of critical section abuse
• Some other minor clarifications
• Add short SEE ALSO

Reviewed by: kib, markj, rpokala, Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39130