docs(storage): redirect users to bucket methods for SignedURL and PostPolicy (#5832)

Author: BrennaEpp

storage/post_policy_v4.go

@@ -239,6 +239,8 @@ func conditionStatusCodeOnSuccess(statusCode int) PostPolicyV4Condition {
 
 // GenerateSignedPostPolicyV4 generates a PostPolicyV4 value from bucket, object and opts.
 // The generated URL and fields will then allow an unauthenticated client to perform multipart uploads.
+// If initializing a Storage Client, instead use the Bucket.GenerateSignedPostPolicyV4
+// method which uses the Client's credentials to handle authentication.
 func GenerateSignedPostPolicyV4(bucket, object string, opts *PostPolicyV4Options) (*PostPolicyV4, error) {
 	if bucket == "" {
 		return nil, errors.New("storage: bucket must be non-empty")

storage/storage.go

@@ -569,6 +569,8 @@ func v4SanitizeHeaders(hdrs []string) []string {
 // access to a restricted resource for a limited time without needing a
 // Google account or signing in. For more information about signed URLs, see
 // https://cloud.google.com/storage/docs/accesscontrol#signed_urls_query_string_authentication
+// If initializing a Storage Client, instead use the Bucket.SignedURL method
+// which uses the Client's credentials to handle authentication.
 func SignedURL(bucket, object string, opts *SignedURLOptions) (string, error) {
 	now := utcNow()
 	if err := validateOptions(opts, now); err != nil {