clarify why its always safe to not append @room at end while parsing

src/editor/deserialize.js

@@ -27,6 +27,9 @@ function parseAtRoomMentions(text, partCreator) {
         if (textPart.length) {
             parts.push(partCreator.plain(textPart));
         }
+        // it's safe to never append @room after the last text
+        // as split will report an empty string at the end if
+        // `text` ended in @room.
         const isLast = i === arr.length - 1;
         if (!isLast) {
             parts.push(partCreator.atRoomPill(ATROOM));