man/man7/mctp.7: Document Linux MCTP support

man/man7/mctp.7: Document Linux MCTP support

This change adds a brief description for the new Management Component
Transport Protocol (MCTP) support added to Linux as of
linux.git bc49d8169aa7 (2021-07-29; "mctp: Add MCTP base").

This is a fairly regular sockets-API implementation, so we're just
describing the semantics of socket(2), bind(2), sendto(2), and
recvfrom(2) for the new protocol.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Message-ID: <20250417-mctp-v3-1-07fff4d26f73@codeconstruct.com.au>
[alx: minor tweaks]
Signed-off-by: Alejandro Colomar <alx@kernel.org>