net: netconsole: add docs for appending netconsole user data

Add a new User Data section to the netconsole docs to describe the
appending of user data capability (for netconsole dynamic configuration)
with usage and netconsole output examples.

Co-developed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>