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.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Xie Yuchen:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Damien Neil:

Patch Set 1:

(1 comment)

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[gopherbot]

This PR (HEAD: 175c55f) has been imported to Gerrit for code review.

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Important tips:

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[gopherbot]

Message from Xie Yuchen:

Patch Set 1:

(2 comments)

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