Updates to pattern types evaluation doc

[yoshuawuyts]

todos

• [x] discuss what happens when pattern types don't subtype (e.g. constraining return types would not be backwards-compatible)
• [ ] discuss the XOR relationship between no_panic and pattern types (e.g. if a function uses pattern types as the input, it could guarantee it won't panic - but if the input isn't constrainted it might panic - which is the behavior of "maybe-pattern-type" functions we've described.)