Update docs for reports endpoint usage

There was a request in the MBO sign-off call to document the private-proejcts endpoint more thoroughly. It is an endpoint that will see infrequent usage, and we want to make sure there is up to date documentation outside of just the openapi spec on usage. While the openapi spec denotes how this is used, we will want to also give more contextual information in an easier-to-consume manner, such as volatile vs non-volatile fields. We will want to denote which fields should be stable like the GitLab user and project IDs, and fields that are more mutable like parent project, EF username, and deletion/creation date. A model for how we might want to present this is the VC tools usage section in the README. It's clean and thorough and is the model of what the ideal documentation is across our products.

Additionally, we will want to add the missing key query parameter that was missed when I was adding the feature last week.