crypto/cipher: fix AEAD.Open documentation nit

crypto/cipher: fix AEAD.Open documentation nit

It mentioned true and false for error values. Instead, just
don't mention the error semantics, as they match normal Go
conventions (if error is non-nil, the other value is
meaningless). We generally only document error values when
they're interesting (where non-nil, non-nil is valid, or the
error value can be certain known values or types).

Fixes #7464

LGTM=agl
R=agl
CC=golang-codereviews
https://golang.org/cl/68440044