Improve documentation that explains how ORA grades are calculated

[jmakowski1123]

Current Open edX documentation does specify that the median is used to determine the peer assessment grade (linked here); however, a quick review of questions in Slack and Discussion Forums makes it clear that there is still some confusion about how this value is derived.

For this reason, I propose we add the following information to the existing documentation in an effort to fully explain how this number is derived:
If there are an even number of peer assessments submitted for a peer reviewed ORA, the average of the two central values is used.