Document platform-specific `SO_REUSEADDR` behavior on `TcpListener::bind`

[lpraneis]

mio's documentation for TcpListener::bind clearly states that the TCP listener will set SO_REUSEADDR on Unix only, as this socket option has a different meaning on Windows.

Tokio's documentation omits the fact that this is Unix-only, which can be confusing for users of the API who haven't consulted the mio documentation.