handle various additionalProperties cases

[Ugzuzg]

Description

Handle various schemas inside additionalProperties correctly (as far as I can tell).

Motivation and Context

Trying to fix issues I have with properly displaying types in additionalProperties.

How Has This Been Tested?

In unit tests and documentation.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes if appropriate.
• [x] All new and existing tests passed.

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[sserrata]

Hi @Ugzuzg, I'm in the process of reviewing this PR. So far, it looks good but I was wondering if you might have an OpenAPI spec/snippet that reproduces the issue you were seeing before the fix?