doc: mention errors thrown by methods called on an unbound dgram.Socket

[mkrawczuk]

• documentation is changed or added
• commit message follows commit guidelines

While working on this I have discovered an inconsistency: given dgram.Socket instance is undbound, calling address() on it results in a system error EBADF being thrown, while calling remoteAddress() results in ERR_SOCKET_DGRAM_NOT_CONNECTED.

Also, calling send() on an unbound socket throws ERR_SOCKET_BAD_PORT which might also be found misguiding.

These I believe are easy fixes. Once the documentation resembles the current state of things I can open another PR introducing a fix to it.

Also mentioned a side effect caused by socket.addMembership() and socket.addSourceSpecificMembership(). Not sure if it is desired.

[addaleax]

I agree, the error system is very inconsistent here, unfortunately … 😕 Thanks for the PR!

[Trott]

The text is fine as is, but if you want to fine-tune it even more, here's one more optional suggestion: Instead of it, I think this method would have been a little better? So "...this method will implicitly bind..." and "This method throws EWHATEVER..." and so forth.

[mkrawczuk]

Yeah, let's not be satisfied with 99% correct if we can be 100% correct with almost no effort.

[Trott]

Landed in 91d9cdf