doc: conn_udp: document that send and receives needs to be called from the same thread

[OlegHahm]

No description provided.

[miri64]

That's rather GNRC specific... so I don't think conn_udp is the right place for that.

[miri64]

(also it's more a bug than a feature, so I would rather fix that instead of documenting faulty behavior ;-))

[OlegHahm]

Well, it is currently the case and confuses users...

[miri64]

I have to solutions in my sleeve:

• the fast and easy fix is to have thread running in gnrc_conn.c that acts as a supervisor for all connections and hands all received packets to the various registered connections (that's basically how I did it for emb6 and lwip, but I did not an extra thread since they are already single threaded and pretty much do all that for you)
• the more sophisticated and memory saving method would be to use core: mbox: initial commit #4919 for that and give every connection it's own mbox. This would require an adaption to gnrc_netreg to also take mboxs into account.

[OlegHahm]

Since neither the one nor the other solution exists, I would rather have this documented.

[miri64]

Will provide one as soon as I find the time.