designs: mctp-kernel: Update tag control details

The tag control mechanism was implemented as ioctls, rather than a
sockopt. Document these using the ALLOC2/DROP2 calls, and update the
sockopt section for consistency.

Change-Id: I177517a90e8e568eb292c6c17dba4ef0fe2eb04b
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>