Add Document inherited members to docs

[bagerard]

Fix the fact that some important members (e.g: to_json, validate, clean, etc) of Document/EmbeddedDocument/etc classes aren't displayed in the Api reference doc because they are defined in the parent (BaseDocument)

This was noticed in #549

[wojcikstefan]

@bagerard, do you have any quick tips on the best way to review such PRs? How do I quickly see the before vs after of the docs?

Apologies – been a long time since I've built/reviewed those 😅

[wojcikstefan]

@erdenezul any tips for reviewing the end result of PRs such as this one?

[erdenezul]

@wojcikstefan nothing particular

[erdenezul]

the validate method's doc including in Document

[wojcikstefan]

Right, but do you build the docs locally via sphinx @erdenezul or is there an easier way to look at them?

[erdenezul]

@wojcikstefan Yes I built it locally

[wojcikstefan]

Alright, I followed the steps from https://github.com/MongoEngine/mongoengine/blob/master/CONTRIBUTING.rst#documentation and built the docs locally as well, thanks!

Updated docs LGTM. I wish our docstrings for Document.clean and Document.get_text_score were clearer, but we can tackle that separately.

Thanks @bagerard & @erdenezul !

[bagerard]

Thanks for including this