errors: document Is and As methods

Add brief descriptions of why one might implement
an Is or As method.

Fixes #33364.

Change-Id: I81a091bf564c654ddb9ef3997e780451a01efb7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/191338


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>