doc: edit documentation that uses "satisfies reads" and "satisfies writes"

doc: edit documentation that uses "satisfies reads" and "satisfies writes"

Make it clear that types that wrap another reader or writer delegate to the wrapped type.

Fixes #7667

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/85720044