Special block transformers and handling should be documented

[tiberiuichim]

We've just had a problem after installing plone.volto, as there are some block fields that are assumed to be of certain types.

For example this line of code assumes a block data structure like:

{ columns: {a, b}, data: {...} )

and I've encountered a block of ours that has a data structure like:

{ columns: "5", data: {...} }