Clarify the form.csrf_enabled deprecation warning

It was speaking about meta.csrf which is pretty much unclear :

- what is this `meta` ?
- suggests that `csrf` is an attribute of `meta` (which is wrong: it is a dict
  key)

flask_wtf/form.py

@@ -79,7 +79,7 @@ def __init__(self, formdata=_Auto, **kwargs):
         if csrf_enabled is not None:
             warnings.warn(FlaskWTFDeprecationWarning(
                 '"csrf_enabled" is deprecated and will be removed in 1.0. '
-                'Set "meta.csrf" instead.'
+                'Pass the "meta" dict arg with a "csrf" key instead.'
             ), stacklevel=3)
             kwargs['meta'] = kwargs.get('meta') or {}
             kwargs['meta'].setdefault('csrf', csrf_enabled)

tests/test_form.py

@@ -132,4 +132,4 @@ class F(FlaskForm):
 
     F(csrf_enabled=False)
     w = recwarn.pop(FlaskWTFDeprecationWarning)
-    assert 'meta.csrf' in str(w.message)
+    assert '"csrf" key' in str(w.message)