Docs update needed: ACTION.md

[mattwelke]

Noticed this while reading the docs:

The payload of the request is stored in the key value. The action should read the field value assuming it is a JSON object (note, not an array, nor a string or number) and parse it.

I know there are a few PRs people were working on to add support for JSON arrays in addition to objects. I'm not involved in that so I don't know exactly when all docs need to be updated, but it looks like this will need to be updated when it's the right time to do so.