net: clarify socket gets closed on drop

[noam-sol]

Add a note like in std::net::TcpListener.

Motivation

I wasn't sure whether tokio TcpListener closes the connection on close, and had to look at the internals to figure out it just wraps a regular TcpListener. Also, I was using the crates warp->hyper and had to look all down the stack to see how the port is managed.

Solution

Added to the docs a note just like in std::net::TcpListener

[Darksonn]

I would say it goes without saying that proper cleanup will happen, but I guess it doens't hurt to add to the docs.

[noam-sol]

@Darksonn I guess so, but when comparing the docs with the std counterpart which states it explicitly, it got me thinking if I'm missing some special cleanup that I need to keep in mind (I'm also new to rust).

Thanks for the quick merge!

[Darksonn]

Thanks for the PR!