MongoDB: document readonly properties and IWM URI options

[alcaeus]

This PR adds documentation for public readonly properties being added in ext-mongodb 2.3.0 as well as the new URI options for Intelligent Workload Management.

[jordikroon]

Since this is still in draft, I will just point these 2 bullets as this will save reviewers a lot of time.

simpara is preferred over para. Could you change this where possible (most of the times).
Also I noticed that informaltable is wrapped in a para. It's fine to remove this outer para.

[alcaeus]

Could you change this where possible (most of the times).

Can you clarify "most of the time"? I have a draft PR up for the changes (see #5517), but I'd like to avoid having to look at every single documentation page to make sure I didn't break anything. I'll update this PR to remove the para wrapping the informaltable elements used for change logs and keep the remaining updates in a separate PR.

[jordikroon]

@alcaeus simpara will pretty much render as <p> tag whereas para will render as <div>.
The technical details can be found here in the docbook manual (see children for what it may contain). But like I said, mostly paragraphs only contain text.