API reference docs problems

[Kircheneer]

I see a couple of problems with the API docs:

• [ ] The API reference wraps in the middle of the page
• [ ] Code snippets don't use monospace
• [x] Not all return types exist in the function signature and sometimes they are manually added in the docstring (#219)

[cardoe]

The docs reference _identifiers, _shortname, and _attributes as lists (see: https://diffsync.readthedocs.io/en/latest/getting_started/index.html#define-your-model-with-diffsyncmodel) but they're actually implemented as tuples. But then when you read how they're used, you really want them to be sets (maybe ordered sets if you really want to preserve that order).