http2/h2c: make the h2c connection docs available in godoc

[xieyuschen]

In the current godoc, we cannot see the description about how "h2c connection" is detected by h2c package so I need to read the code to see. Actually this part has been mentioned already but under an un-exported structure.

This PR tries to move it to the package level comments, so we could view it via the go doc.