Explain wrapping and how to use Is and As in the package doc.

Explain "chain" in Is and As.

Updates #33364.

Change-Id: Ic06362106dbd129e33dd47e63176ee5355492086
Reviewed-on: https://go-review.googlesource.com/c/go/+/188737


Reviewed-by: Rob Pike <r@golang.org>