tipc: Update comments in message header include file

Removes comments in TIPC's message header include file that are
outdated and/or unnecessary. Also introduces short comments (or
supplements existing ones) to better describe several set of existing
symbolic constants.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>