Improve documentation of accounts

[vyasr]

trafficstars

Feature description

Right now submission on a specific environment uses a template filter to pull the account from the environment's namespace in the config. This behavior is undocumented, but if an environment template makes use of an account it will print a message to the user indicating that an account can be set (in some cases, must be set). We should add this to the docs. More generally, we should probably document any other config variables users may want to set; similarly to how directives work, we may want a set of config variables that have some reliable effect on submission templates.

[kidrahahjo]

@vyasr can I try to document these? This might help me improve my knowledge of cluster submission.

[csadorf]

@kidrahahjo Yes, thank you!