net/http: document Request.Context's lifetime again

[bradfitz]

It was originally documented as closing when the peer closed for Go 1.7, but then we removed that part of the docs when the implementation wasn't complete.

The implementation is complete in Go 1.8, so we need to add the docs back.

[bradfitz]

(Original revert was 15db365)

[gopherbot]

CL https://golang.org/cl/33769 mentions this issue.