rc.subr.8: Document changes to load_rc_config

Since e27961a496322e2590da29f3c5fe710b269e6754, load_rc_config does not
require a service name as its first argument. This change was documented
in the rc.subr script in 0b9c2e7ac58caac0413aa36749e4c212514d142d. Let's
update the manual page as well.

MFC after: 3 days