Some documentation is missing

The OTIO documentation here https://opentimelineio.readthedocs.io/ is incomplete. There are some classes, functions, and concepts that are missing documentation entirely, or need additional information. We would love for a volunteer to read/skim through the documentation as-is and make a checklist of areas that are missing.

Examples:
- Item.1 is missing a doc string: https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-serialized-schema.html#item-1
- MediaReference.1 is missing a doc string: https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-serialized-schema.html#mediareference-1
- RationalTime.1 has docs, but they are too short and there's a math formatting problem: https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-serialized-schema.html#rationaltime-1
- etc.

If we had a detailed checklist, then we could more easily ask people to fill in those gaps & track our progress on improving the documentation.

Also: fixes/additions to the documentation content itself are quite welcome.

Start by skimming the OTIO documentation at https://opentimelineio.readthedocs.io/, using the serialized-schema pages for Item.1, MediaReference.1, and RationalTime.1 as initial entry points. Record missing or insufficient class, function, and concept documentation in a detailed checklist; done means the gaps are identified clearly enough for others to address, with content fixes welcome.

Written by the indexing model from the issue text.