crypto/tls: document ConnectionState.NegotiatedProtocol more clearly
crypto/tls: document ConnectionState.NegotiatedProtocol more clearly

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 <c22b5f9178342609428d6f51b2c5af4c0bde6a42@filippo.io>
Reviewed-by: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

Jigsaw-Code/utls