Update to API docs

[nbbaier]

I've updated the API documentation in the following way:

• Added descriptions from the OpenAPI Spec to each endpoint in the docs
• Updated the example for POST /v1/vals to reflect that it is possible to include readme and privacy in the request body
• Added the PUT /v1/vals endpoint and added an example for it
• Added the GET /v1/me/references endpoint and example for it
• Changed handle where it was used in the API docs to better reflect what the path parameter is (it was used for both val_name and username)

Partially resolves #100

[stevekrouse]

Thanks @nbbaier!!