docs: link from Connection to handshake

[WhyNotHugo]

When reading the documentation for Connection it is not entirely obvious how instances of this type are obtained. Add a helpful link, mostly for those less familiar.

[tottoto]

Although I think these links are helpful, having independent sections sounds a bit excessive. How about adding them as inline explanations, similar to the server's documentation?

• https://docs.rs/hyper/1.4.1/hyper/server/conn/http1/struct.Connection.html
• https://docs.rs/hyper/1.4.1/hyper/server/conn/http2/struct.Connection.html

[WhyNotHugo]

Yeah, the new section is pointless. Updated.

[WhyNotHugo]

The explicit link was also unnecessary; the handshake function is in the same module.

[tottoto]

Thank you!