Updated docs for slot type any

[ganeshviyer]

Proposed changes:

• The update is regarding insufficient documentation for slot type "any" where the the slot can carry a JSON or a list value. This is not compatible when performing the migration or when generating new stories through rasa interactive. The resulting training file (stories.yml) breaks any subsequent training.
• The corresponding issue has been discussed here in the forums - https://forum.rasa.com/t/the-slot-type-any-does-not-seem-to-work-correctly/38674/3
• The issue on Github has been raised here - https://github.com/RasaHQ/rasa/issues/7683 and is still open.

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)

[CLAassistant]

All committers have signed the CLA.

[sara-tagger]

Thanks for submitting a pull request 🚀 @JustinaPetr will take a look at it as soon as possible ✨

[indam23]

@ganeshviyer Sorry I couldn't tag you in the thread, not sure why - does the discussion make sense to you? Would you like to take care of moving the info up the page and adding a note?

[stale[bot]]

This PR has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.