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 
This header is used by ownCloud to prevent XSS attacks, basically we instruct the browser to not execute any inline JavaScript.

Add a note that this should NOT get changed without a *very* good reason and ownCloud uses this to prevent XSS attacks. - Also note that changing this from the default value might lead to broken features in case the standard ownCloud policy is changed