firewire: Add deprecation notice

[emaste]

The 15 planning doc has a note

do we strip out disk support sooner has a GIANT locked CAM driver

Maybe we'll remove GIANT-locked components (across the tree) for FreeBSD 16 and the non-disk firewire support will remain beyond 16.

[imp]

I thought we'd killed all GIANT CAM SIMs. If not, we should.

[brooks]

👍 to the idea, possible text change

share/man/man4/firewire.4

55–56

[emaste]

I thought we'd killed all GIANT CAM SIMs.

I was only going based on the 15 planning doc which says "do we strip out disk support sooner has a GIANT locked CAM driver" -- maybe that was