Docs: Add warning about some properties being ignored in GtkPaned

Hi, first time contributor here, this is purely a documentation change.

I recently spent a while debugging an issue with GtkPaned. It was not resizing its children correctly.

I got some help from some folks in discourse, and have resolved my issue: https://discourse.gnome.org/t/paned-resizes-the-wrong-child/19699/3

They highlighted that the issue has come up before: issue #5500

Previous proposals to change the behaviour or add a runtime warning seem to be unpopular. In the absence of either of these, I would like to at least see this documented in the main reference documentation, so that's why I am proposing this change.

I have not used gi-docgen before. Is there a better way to have a link to a warning? (I've used some raw html to put an anchor tag in to refer to from other pages, perhaps I've missed a gi-docgen feature for this?)

Thanks :)