DOC: Improve documentation for config refresh

Document that a value of "-1" may be used for the
CONFIG_REFRESH_FREQUENCY to indicate that the config
file should only be read once, at the very first
time-step CosmoTools is invoked.

From: George Zagaris <george.zagaris@kitware.com>

git-svn-id: https://svn.alcf.anl.gov/repos/DarkUniverse/hacc/trunk/cosmotools@2157 952bdfbe-fa2e-45c0-a9ae-ac90d4a62daf
Merge branch 'svn/trunk'