Improve docs about plugin registration

[KoBeWi]

Closes godotengine/godot-proposals#195
The docs for EditorExportPlugin were filled, but they didn't mention the most important thing - that you need to register the plugin for any method to be called.

I also made the same changes to other similar plugins, unifying it a bit.

[YuriSizov]

We can take this opportunity to unify the descriptions even further. But otherwise seems good.

[YuriSizov]

Pluralizing cross-references like that often looks ugly and doesn't help with localizations, but it's not critical right now. 🙃

[akien-mga]

Thanks!

[akien-mga]

This could use a dedicated cherry-pick PR, there's a number of conflicts and renamed classes to handle.