Add docblock and fix the default value

Author: Marco Pereirinha

cloudinary-image-management-and-manipulation-in-the-cloud-cdn/php/class-media.php

@@ -633,7 +633,19 @@ public function attachment_url( $url, $attachment_id ) {
 		if ( false !== $previous_url ) {
 			return substr( $url, $previous_url );
 		}
-		if ( ! doing_action( 'wp_insert_post_data' ) && false === $this->in_downsize && ! apply_filters( 'cloudinary_doing_upload', '__return_false' ) ) {
+		if (
+			! doing_action( 'wp_insert_post_data' )
+			&& false === $this->in_downsize
+			/**
+			 * Filter doing upload.
+			 * If so, return the default attachment URL.
+			 *
+			 * @param bool Default false.
+			 *
+			 * @return bool
+			 */
+			&& ! apply_filters( 'cloudinary_doing_upload', false )
+		) {
 			if ( $this->cloudinary_id( $attachment_id ) ) {
 				$url = $this->cloudinary_url( $attachment_id );
 			}