docs(forms): add more to the top of doc warning about bootstrap

Author: naomiblack

public/docs/ts/latest/guide/forms.jade

@@ -2,9 +2,11 @@ include ../_util-fns
 
 .alert.is-important
   :marked
-    This guide is using the new forms API. 
+    This guide is using the new forms API. To use this API, you must opt in by adding special
+    providers to your bootstrap file (see the Bootstrap seection below).
 
-    The old forms API is deprecated, but we still maintain a separate version of the guide using the deprecated forms API <a href='/docs/ts/latest/guide/forms-deprecated.html'>here</a>.
+    The old forms API is deprecated, but we still maintain a separate version of the guide using
+    the deprecated forms API <a href='/docs/ts/latest/guide/forms-deprecated.html'>here</a>.
 
 :marked
   We’ve all used a form to login, submit a help request, place an order, book a flight,