net/http: explain more how Pusher.Push works

This will clarify that the resources are not completely pushed yet when `Push` returns and that it starts a separate goroutine. This might be implementation dependant but as I believe there is currently only one implementation it should be added to the documentation of the interface which most people will look up first.

Change-Id: Id151c5563fd0c4e611eb1d93b4f64bf747ddf6d4
GitHub-Last-Rev: 1f46eb9a081f4529abc04026589ffd463be579b3
GitHub-Pull-Request: golang/go#25025
Reviewed-on: https://go-review.googlesource.com/c/108939


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>