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>

akutz/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,codestation/go,CAFxX/go,Cofyc/go,polyverse-security/go,cloudflare/go,nawawi/go,derekmarcotte/go,CAFxX/go,polyverse-security/go,CAFxX/go,josharian/go,codestation/go,codestation/go,polyverse-security/go,polyverse-security/go,golang/go,vsekhar/elastic-go,Cofyc/go,vsekhar/elastic-go,linux-on-ibm-z/go,linux-on-ibm-z/go,josharian/go,josharian/go,josharian/go,codestation/go,derekmarcotte/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,ImJasonH/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,codestation/go,cloudflare/go,golang/go,nawawi/go,ImJasonH/go,ImJasonH/go,linux-on-ibm-z/go,derekmarcotte/go,golang/go,vsekhar/elastic-go,vsekhar/elastic-go,derekmarcotte/go,golang/go,akutz/go,linux-on-ibm-z/go,golang/go,akutz/go,cloudflare/go,derekmarcotte/go,nawawi/go,CAFxX/go,nawawi/go,codestation/go,linux-on-ibm-z/go,golang/go,vsekhar/elastic-go,ImJasonH/go,golang/go,akutz/go,polyverse-security/go,derekmarcotte/go,cloudflare/go,ImJasonH/go,Cofyc/go,golang/go,ImJasonH/go,polyverse-security/go,codestation/go,josharian/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,akutz/go,derekmarcotte/go,cloudflare/go,ImJasonH/go,cloudflare/go,nawawi/go,akutz/go,linux-on-ibm-z/go,cloudflare/go,christopher-henderson/Go,vsekhar/elastic-go,akutz/go,Cofyc/go,linux-on-ibm-z/go,ImJasonH/go,josharian/go,Cofyc/go,nawawi/go,josharian/go,polyverse-security/go,codestation/go,Cofyc/go,Cofyc/go,polyverse-security/go