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

src/pkg/crypto/cipher/gcm.go

@@ -30,9 +30,9 @@ type AEAD interface {
 
 	// Open decrypts and authenticates ciphertext, authenticates the
 	// additional data and, if successful, appends the resulting plaintext
-	// to dst, returning the updated slice and true. On error, nil and
-	// false is returned. The nonce must be NonceSize() bytes long and both
-	// it and the additional data must match the value passed to Seal.
+	// to dst, returning the updated slice. The nonce must be NonceSize()
+	// bytes long and both it and the additional data must match the
+	// value passed to Seal.
 	//
 	// The ciphertext and dst may alias exactly or not at all.
 	Open(dst, nonce, ciphertext, data []byte) ([]byte, error)