Improve documentation of compset forcing time period

Motivated by this forum thread:

https://bb.cgd.ucar.edu/cesm/threads/how-to-set-the-initialization-time-in-cesm.8594/

From a quick look through our documentation, I don't see clear documentation / guidance on the compset forcing time period, its relationship to RUN_STARTDATE and other settings, etc. Unless I'm missing something, I think we should add some better documentation on this.