add use_next_cylc docs (#181)

source/Reviewers/nightlytesting.rst

@@ -33,7 +33,7 @@ Rose-stem testing is controlled by a config file located at ``~/testing_configs.
 
 * ``monitoring``: Controls whether to launch the monitoring script on this suite. If true, the monitoring script will be launched at 06:00.
 
-* ``cylc_version``: Controls which cylc_version to use. The suite is now set up to use primarily cylc8 with some suites being launched at Cylc7 for the UM and Jules.
+* ``cylc_version``: Controls which cylc_version to use. The suite is now set up to use primarily cylc8 with some suites being launched at Cylc7 for the UM and Jules. Can be any string beginning "7" or "8" that is a valid cylc install at the site, such that ``export CYLC_VERSION=<cylc_version>`` works.
 
 The cronjobs are installed by running the ``generate_test_suite_cron.py`` script with the ``--install`` command line option. This script is stored in the SimSys_Scripts github repo. It will read a config file, generate a cron file and then install the cronjobs from all files with extension .cron in a specified location. The script has the following command line arguments: