Document Problem responses in API docs

Quoting @lognaturel in Slack:

> We don’t currently document detailed error codes. It looks like the .1 variants are the only ones documented. Those codes can be really useful for clients and I’d like to use them in `pyodk`. … my understanding is that we could document 404.1, 404.2, etc separately with API Blueprint because there’s no semantics around HTTP error codes in the spec, the definitions are using standard data structures

The goals of this issue are:

- Document all Problem **codes**.
- For .1 variants only: Document the structure of the `details` property.

This won't fully document everything about Problem responses, but it would be a great first step.

No file is named in the issue. Start by locating the API Blueprint definitions for Problem responses and the currently documented .1 variants. Enumerate all Problem codes and document the details structure for .1 variants; the work is done when the API docs cover those codes and structures.

Written by the indexing model from the issue text.