js: Add JSDoc to exported functions and fix incorrect JSDoc formatting

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1499>

js/gdm/vmware.js

@@ -46,6 +46,9 @@ var VmwareCredentialsManager = class VmwareCredentialsManager extends Credential
     }
 };
 
+/**
+ * @returns {VmwareCredentialsManager}
+ */
 function getVmwareCredentialsManager() {
     if (!_vmwareCredentialsManager)
         _vmwareCredentialsManager = new VmwareCredentialsManager();