SMB3.1.1: update comments clarifying SPNEGO info in negprot response

Trivial changes to clarify confusing comment about
SPNEGO blog (and also one length comparisons in negotiate
context parsing).

Suggested-by: Tom Talpey <tom@talpey.com>
Suggested-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>