text: Title: register_streamfield_block not documented?
  Body: Hi guys - new to wagtail-grapple (great product, btw).
  
  I am seeing references to register_streamfield_block in several places in the example, but no definition of exactly what it is, or when it should be used.  Seems like it should be documented on the Decorators page.
  
  IF I have nested StructBlocks inside StreamFields, and the top-level is just a StreamField which contains those blocks, do I need to use the decorator on the nested blocks?
  
  