Review and document various env variables

[stronk7]

Detected @ #268 , there are a good number of environmental variables (with or without configuration option alternative) that aren't documented anywhere. Example: MOODLE_START_BEHAT_SERVERS.

We need to find all them and decide the best way to document them (can be in CLI help, in specialised docs page, like the APP one) or any other alternative we can imagine.

Ciao :-)