Update AuthenticationProvider Documentation
Please take a look at my comment at https://www.phpbb.com/community/posting.php?mode=quote&f=461&p=14577286#pr14577276

I created my authentication provider and named it eg. "myext.authentication.MyAuthenticationProvider".
But my authentication provider will not be found.
The authentication provider service name has to start with "auth.provider." e.g. "auth.provider.MyAuthenticationProvider".
That check is hard coded in phpBB file "auth/provider_collection" method "get_provider" of v3.1.10.
If service name does not start with "auth.provider" the default provider "db" will be used.

Can you please verify?

Greetz Anthony

Proper highlighting
Include "<service name>" in highlighting.

Proper highlighting
Highlighting "auth.provider." with double `` as requested by VSEphpbb (https://github.com/phpbb/documentation/pull/101#discussion_r92210852)

Reorder
As requested by VSEphpbb (https://github.com/phpbb/documentation/pull/101#pullrequestreview-12370193) i move the missing information.