Improve documentation of compset forcing time period

[billsacks]

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.

[alperaltuntas]

Just noting that the lack of time period documentation is something I also stumbled upon while working on the visualCaseGen GUI. I'd like to provide more guidance to users for picking the right time period, but it is unclear to me when it matters (does it only matter when ATM and/or LND is active?), or which constraints and guidance to provide when a custom compset gets built.