Add deprecation notice to mn(4)

Add deprecation notice to mn(4)

Sync serial (T1/E1) interfaces are largely irrelevant today and phk
confirms this driver is unnecessary in review D23928.

This leaves ce(4) and cp(4) in the tree.  They're likely not relevant
either, but glebius contacted the manufacturer and those devices are
still available for purchase.  At glebius' suggestion leave them in
the tree as long as they do not impose a maintenace burden.

Approved by:	phk
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation