Add a note to CPF UI that display of CPFs on viewtopic is conditional on the respective load setting

While testing http://tracker.phpbb.com/browse/PHPBB3-9084:

(03:54:07) nw-: If this option is enabled, the field will be displayed in the mini-profile on the topic screen. - i don't have this
(03:54:21) nw-: i made a required and a not required field and neither is displayed in viewtopic
(03:56:09) nw-: must be a load setting
(03:56:27) nw-: Display custom profile fields on topic pages:
(03:56:38) nw-: there is no way a beginning admin would know this
(03:56:54) nw-: heck it's amazing i know this
(03:58:43) rxu: funny
(03:59:08) rxu: you sorted it?
(03:59:36) nw-: yes and a ticket coming up
(03:59:47) nw-: it's not actually funny
(03:59:58) nw-: i must have been searching for it last time
(04:00:11) nw-: i wouldn't be surprised if someone else pointed out the load setting part