ConnectionState.NegotiatedProtocol's documentation implies that it will
always be from Config.NextProtos. This commit clarifies that there is no
guarantee.

This commit also adds a note to
ConnectionState.NegotiatedProtocolIsMutual, making it clear that it is
client side only.

Fixes #18841

Change-Id: Icd028af8042f31e45575f1080c5e9bd3012e03d7
Reviewed-on: https://go-review.googlesource.com/35917
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>