Add explenation about RoleVoter prefix

[Patrick-Beuks]

This took me a hot minut to debug why something like a SCOPE_ prefix did not work out of the box.

Symfony's RoleVoter uses a prefix for IsGranted to be able to vote. By default this is ROLE_. The easiest fix would be to add information about this to the documentation in the two places below. Where it is stated that it needs to start with the prefix defined for RoleVoter

A better fix would be to find what this prefix is and add it, if it is not already the prefix.

oauth2-server-bundle/docs/index.md

Lines 102 to 103 in e6eaf34

	# Set a custom prefix that replaces the default 'ROLE_OAUTH2_' role prefix
	role_prefix: ROLE_OAUTH2_

oauth2-server-bundle/src/DependencyInjection/Configuration.php

Lines 32 to 34 in f9f55a9

	->scalarNode('role_prefix')
	->info('Set a custom prefix that replaces the default \'ROLE_OAUTH2_\' role prefix')
	->defaultValue('ROLE_OAUTH2_')

[fastmanu]

Same here