[rb-general] Clarification re SOURCE_BUILD_EPOCH and clamping

[Dan Kegel]

https://reproducible-builds.org/specs/source-date-epoch/
says
"Upstream build processes MUST use this variable for embedded
timestamps in place of the "current" date and time."
but https://github.com/rpm-software-management/rpm/pull/144 and others
interpret that as not a mandatory timestamp, but rather a limit on the
maximum timestamp.

The spec doesn't seem to allow that, does the spec need updating?