cleanup: clarify why mktime() can be mixed with gmtime() (#4602)
cleanup: clarify why mktime() can be mixed with gmtime() (#4602)

This does not change any behavior, but it clarifies how we force
`std::mktime()` to ignore DST transitions, which allows it to be
used as the inverse to `std::gmtime()` under the assumption that
the "standard UTC offset" does not change.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp