DOUT_S_SAVE_INTERIM_RESTART_FILES needs to be documented

The behavior of DOUT_S_SAVE_INTERIM_RESTART_FILES being set to false and removing interim restart files at the end of the run needs to be documented. Two locations have been proposed (and it might be good to have it documented in both places):
1) In the env_run.xml, one of the REST_XX variables could put in a caution that DOUT_S_SAVE_INTERIM_RESTART_FILES=TRUE needs to be set if a user wishes interim files to remain.
2) The cime User's Guide in the restart section needs a cautionary paragraph like:
It should be pointed out that the default is to only keep the final restart files upon job completion. If a user wants to keep all of the interim restart files, then ``DOUT_S_SAVE_INTERIM_RESTART_FILES`` must be set to TRUE.