Document that UdpSocket::recv and recv_from do not read from the buffer

```rust
fn recv(&self, buf: &mut [u8]) -> Result<usize>;
fn recv_from(&self, buf: &mut [u8]) -> Result<(usize, SocketAddr)>;
```

It is important for these to document that they only write to the buffer, not read from it, because that means it is safe to pass an uninitialized buffer. It would be wasteful to construct a buffer, initialize it to contain all 0 bytes, and pass it to UdpSocket.