Improve UdpSocket documentation

I tried working with `UdpSocket` and ran into `EINVAL` errors with no
clear indication of what causes the error. Also, it was uncharacteristically
hard to figure this module out, compared to other Rust `std` modules.

1. `send` and `send_to` return a `usize`
This one is just clarity. Usually, returned `usize`s indicate that the
buffer might have only been sent partially. This is not the case with
UDP. Since that `usize` must always be `buffer.len()`, I have documented
that.

2. `bind` limits `connect` and `send_to`
When you bind to a limited address space like localhost, you can only
`connect` to addresses in that same address space. Error kind:
`AddrNotAvailable`.

3. `connect`ing to localhost locks you to localhost
On Linux, if you first `connect` to localhost, subsequent `connect`s to
non-localhost addresses fail. Error kind: `InvalidInput`.

Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>

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