Add new response 'eligibility' field to docs

[becky-gilbert]

We are adding a new 'eligibility' field to the response data available to researchers. We should add this to the documentation, and be sure to (a) explain what the different levels mean, and (b) explain when/how these values are determined.

Here are the relevant changes:
lookit/lookit-api#1292
lookit/lookit-api#1300