doc(storage): minor fix to PostPolicyV4 docs (#8172)

These referred to signed URLs instead of post policies in a couple places; someone was confused by this recently.

storage/post_policy_v4.go

@@ -32,7 +32,7 @@ import (
 // Please see https://cloud.google.com/storage/docs/xml-api/post-object
 // for reference about the fields.
 type PostPolicyV4Options struct {
-	// GoogleAccessID represents the authorizer of the signed URL generation.
+	// GoogleAccessID represents the authorizer of the signed post policy generation.
 	// It is typically the Google service account client email address from
 	// the Google Developers Console in the form of "[email protected]".
 	// Required.
@@ -85,7 +85,7 @@ type PostPolicyV4Options struct {
 	// Exactly one of PrivateKey or SignRawBytes must be non-nil.
 	SignRawBytes func(bytes []byte) (signature []byte, err error)
 
-	// Expires is the expiration time on the signed URL.
+	// Expires is the expiration time on the signed post policy.
 	// It must be a time in the future.
 	// Required.
 	Expires time.Time