Document undocumented fields for List Group milestones REST API

Proposal

I originally created Add containing_date to List Group milestones RE... (#480155 - closed) • Adam Cohen • 17.4 to bring parity between the Group.milestones GraphQL endpoint and the List group milestones REST API endpoint, however, after further investigation, it turns out the containing_date field is currently supported by the List group milestones endpoint, it just isn't documented.

Other fields that are supported but not documented are the following:

• search_title
• start_date and end_date

The purpose of this issue is to document these fields in the List group milestones REST API endpoint.