Update docs on study prior participation criteria

[becky-gilbert]

We need to add/clarify a few things in the docs to go along with the changes to the 'must have/not have participated' study eligibility criteria:

• lookit/lookit-api#1302
• lookit/lookit-api#1306

Specifically, the docs should clarify the following:

• How prior study participation is checked: we count any non-empty responses to internal studies as having participated, even if the participant does not finish. We also count any attempt to do external studies (since we have no way of knowing if the child completed an external study).
• If the researcher wants to prevent a child from doing their study more than once, they can add the study itself to the 'must not have participated' list, keeping in mind that the child will not be considered eligible if they start the study but don't finish. We can point researchers to alternatives if they want to implement the study participation criteria in a different way (e.g. get past sessions from protocol generator to check that they haven't completed the current study).
• If there are multiple studies in the 'must have participated' list for study eligibility, the child must have participated in ALL of studies in the list. That is, in order to be eligible, they must have participated in study1 AND study2 AND ... studyN.
• If there are multiple studies in the 'must not have participated' list for study eligibility, the child is only eligible if they have not participated in ANY of the studies in the list.
• If the child is not eligible due to the study's prior participation requirements, they can still choose to participate in the study, but they will see a 'red text' warning below the 'Participate Now' button when the ineligible child is selected.