net/http: http.Request.Context doc fix

net/http: http.Request.Context doc fix

The comment on http.Request.Context says that the context
is canceled when the client's connection closes even though
this has not been implemented. See #15927

Change-Id: I50b68638303dafd70f77f8f778e6caff102d3350
Reviewed-on: https://go-review.googlesource.com/23672
Reviewed-by: Andrew Gerrand <adg@golang.org>