disabled shell docs

[johrstrom]

we've had 2 discourse topics around the same setting.

https://discourse.openondemand.org/t/bin-true-account-access-to-ondemand/2990
and
https://discourse.openondemand.org/t/expiring-users-at-a-future-date/2976

This ticket is to

1. give prominence to this setting - not quite sure how (maybe in customization?)
2. update the page that actually holds the setting as - restrict starting a per-user NGINX process as a user with the given shell (the current help line) seems to be confusing.
   Here's a possible update

disabled_shell means anyone with this shell will be disabled from starting a PUN.

or

disabled_shell means any account with this shell will is disabled on the system and 
will not be able to use Open OnDemand.