bridge.4: describe new MTU behaviour

1865ebfb12 changed if_bridge to have it change the MTU on newly added
interfaces to match the if_bridge MTU, rather than rejecting them for
having an incorrect MTU.

Update the man page to reflect this, as pointed out by woodsb02.

Reviewed by: woodsb02
Differential Revision: https://reviews.freebsd.org/D36481