build.7: update documentation around world-clean options

[kevans]

Summary

build(7) doesn't seem to have made it across the NO_CLEAN -> MK_CLEAN
transition, so do this now. The former is still accepted, but it will
issue a notice that one can use WITHOUT_CLEAN now. It is preserved in
this documentation so that one can still quickly tell that they're
equivalent if they're curious about some current use of the option.

Additionally add some discussion about the recently added
.clean_build_epoch to explain why a clean might be observed despite use
of the WITHOUT_CLEAN option.

[markj]

Thank you.