ALLOW_CDN_EXPLAIN: Incomplete and imprecise description

Concerns the description of the switch "APC > GENERAL > Load settings > Allow usage of third party content delivery networks:".

• The text gives the impression that the choice of the switch state is a personal preference of individual administrators. In fact, this is a matter of local privacy laws.
• Missing mention that the switch also controls the integration of Font Awesome, part of phpBB's GUI and extensions.

PR follows

Reference:
A topic on phpBB.de triggered a closer examination of the switch: https://www.phpbb.de/community/viewtopic.php?t=246522