Add comment regarding presence of '/documents' to fromQueryPath(). (#1474)

Author: mikelehen

packages/firestore/src/remote/serializer.ts

@@ -345,6 +345,10 @@ export class JsonProtoSerializer {
 
   fromQueryPath(name: string): ResourcePath {
     const resourceName = this.fromResourceName(name);
+    // In v1beta1 queries for collections at the root did not have a trailing
+    // "/documents". In v1 all resource paths contain "/documents". Preserve the
+    // ability to read the v1beta1 form for compatibility with queries persisted
+    // in the local query cache.
     if (resourceName.length === 4) {
       return ResourcePath.EMPTY_PATH;
     }