text: Title: Form -> FlaskForm rename breaks custom metaclasses
  Body: When using a custom metaclass you usually subclass `FormMeta`. This fails thanks to the different metaclass used to show the deprecation warning (#249):
  
  > TypeError: Error when calling the metaclass bases
  >     metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
  
  It's an easy fix (not using the deprecated `Form` class) but still, I think this should at least e mentioned in the docs.
  
  cc @davidism 
  