[Reviewing CLA] Adding note about returning on `extract_` methods

[josejulio]

Proposed changes: Adding additional information i saw by observing/trying Rasa actions when validating forms.

This is something I saw by observing, I'm not enterly sure that this is the default behavior, but when I return only {} the message was processed as other intent e.g.

Bot: Where are you hosting your applications?
Me: I don't know
Bot: Bye! # <--- This is what I wanted to avoid and did so by returning {"the-slot": None}
Bot: You can be hosting your applications on-premise or the cloud.
Bot: Where are you hosting your applications?

Status (please check what you already did):

• [ ] added some tests for the functionality
• [x] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)