libc: Improve description of mktime() / timegm().

• Mention that mktime() and timegm() set errno on failure.
• Correctly determining whether mktime() / timegm() succeeded with arbitrary input (where -1 can be a valid result) is non-trivial. Document the recommended procedure.

PR: 277863
MFC after: 1 week
Reviewed by: pauamma_gundo.com, gbe
Differential Revision: https://reviews.freebsd.org/D44503