reflect: explain why convertible or comparable types may still panic

Conversions of slices to arrays may panic since the slice is too short.
Comparibility of interfaces may panic since the underlying value is incomparable.

This is a follow-up to CL 301652

Change-Id: Ia5d84a6e556a7b82c39add4be93ed7463e63cc8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/327589
Trust: Joe Tsai <joetsai@digital-static.net>
Trust: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/reflect/type.go

@@ -107,10 +107,14 @@ type Type interface {
 
  // ConvertibleTo reports whether a value of the type is convertible to type u.
  // Even if ConvertibleTo returns true, the conversion may still panic.
+ // For example, a slice of type []T is convertible to *[N]T,
+ // but the conversion will panic if its length is less than N.
  ConvertibleTo(u Type) bool
 
  // Comparable reports whether values of this type are comparable.
  // Even if Comparable returns true, the comparison may still panic.
+ // For example, values of interface type are comparable,
+ // but the comparison will panic if their dynamic type is not comparable.
  Comparable() bool
 
  // Methods applicable only to some types, depending on Kind.