API documentation cleanup.

Basically:

* use `()` for all the functions.
* Expand to 80 chars width.
* "i/o" => "I/O".
* "unix" / "Unix" => "UNIX".
* More consistency.
* Clarify that, upon EOF, the callee must call to uv_close() or uv_read_stop().
we should add a note here mentioning that 0 size would also mean an empty datagram, in case addr is not NULL.