new config_sample_php manual page

Eyeballs wanted! This is the authoritative config_sample_php reference so it needs to be good. 

The very last item on the page, Custom CSP policy, needs more explanation. The whole page needs review.

Two items are missing from config_sample_php.rst:
version
instanceid

I tried 'singleuser' => true, and nothing changed. All of my non-admin users could still log in and do stuff.

@PVince81 @danimo @LukasReschke @MTRichards @craigpg 
Yes. This setting isn't used any more.
Seems I forgot to check it when moving the code to the JS side... Do we still want the flag @karlitschek ?

Point is that this check is not blocking any more as it was before as it's only done on the admin page through ajax, so will not disturb the admin's workflow.