Add docs for feature_ftue_dms flag

This should hopefully be short-lived.

For matrix-org/matrix-react-sdk#3792

Author: turt2live

docs/labs.md

@@ -81,3 +81,10 @@ that downloads, stores, and indexes room messages for E2E encrypted rooms.
 
 The existing search will transparently work for encrypted rooms just like it
 does for non-encrypted.
+
+## New DM invite dialog (`feature_ftue_dms`)
+
+An improved dialog for inviting users. Once finished, this will replace both the
+DM creation dialog and the 'invite user' dialog. This is a work in progress and
+as such not all functionality may be present until the feature replaces today's
+UX.