Write better documentation for UDP handling

[marcelstoer]

Explain the main ideas in the "net.udpsocket Module" chapter right before the documentation of the individual functions.
Furthermore, the sentence "received callback have port and ip after data argument." isn't correctly formatted (bold?).

[marcelstoer]

@djphoenix I added a few notes, pls see http://nodemcu.readthedocs.io/en/latest/en/modules/net/#netudpsocket-module

I would appreciate if you could add a standalone example, maybe below the net.udpsocket:send() documentation? That one is really terse.

[marcelstoer]

Furthermore, net.udpsocket:send() is lacking important parameter documentation. I figured it out through some trial and error.

I kindly ask someone who knows that stuff to now also review 0ca4b6b and then close this issue if all is well.

This and #1771 are the last pending issues for the next release.

[djphoenix]

@marcelstoer good work!

[eyaleb]

re #343.

Now that we use SDK 2.0.0 in dev, is there a reliable way to send a UDP packet and then dsleep after it is sent (send queue is empty, naturally with UDP there is no concept of the packet delivered)?

I am still unhappy about the hack I use, which may not work if a longer delay is needed due to other SDK (or other) changes in the future.

[djphoenix]

There is no callback for actual UDP send (that means that packet was passed to radiomodule and this have passed it on air).

[eyaleb]

@djphoenix and I guess no way to check the status of the radio send buffer (is empty)?

[djphoenix]

@eyaleb unfortunately, no way.

[pjsg]

Also, note that since 802.11 is an acknowledged protocol, there can be a significant time delay before the frame gets acknowledged.

I took a quick look, and I couldn't find an upper bound for the time that the MAC might take to send a data frame.