Docs: remove reference to processes that don't exist yet

[nickmoreton]

Question:

There's a filter that will convert it to a block type/string that can be used to build the block type in the block builder.

Is there? I don't understand this sentence.

_Originally posted by @nimasmi in https://github.com/torchbox/wagtail-wordpress-import/pull/36#discussion_r731032322_

[nickmoreton]

Fixed #49