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>