Document the three GODEBUG environment variables in the package doc.

Updates the bundled http2 to x/net git rev 415f1917
for https://golang.org/cl/18372.

Fixes #13611

Change-Id: I3116c5d7de70d3d15242d7198f3758b1fb7d94b9
Reviewed-on: https://go-review.googlesource.com/18373Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>