Add jsdoc for prepareAttachmentTextForUpdate()

apps/app/src/server/util/slack.js

@@ -24,6 +24,12 @@ const prepareAttachmentTextForCreate = function(page, siteUrl) {
   return convertMarkdownToMarkdown(body, siteUrl);
 };
 
+/**
+ * Return diff with latest revisionBody
+ * @param {IPageHasId} page
+ * @param {string} siteUrl
+ * @param {IRevisionHasId} previousRevision
+ */
 const prepareAttachmentTextForUpdate = function(page, siteUrl, previousRevision) {
   if (previousRevision == null) {
     return;