Add default endpoint descriptions

Writing endpoint descriptions is important, but sometimes we don't do this.
This is fine for default Swagger UI, but some more fancy versions (like https://github.com/jensoleg/swagger-ui) don't display the list of endpoints nicely when there are no descriptions:

So maybe we should provide some mechanism to generate at least some sort of descriptions
(semi-)automatically.

The issue names no files, tests, or entry points. Start by reviewing the existing Swagger generation behavior and the five-comment discussion; done means defining and implementing a mechanism that supplies useful endpoint descriptions when explicit descriptions are absent, including behavior suitable for alternate Swagger UIs.

Written by the indexing model from the issue text.