Child properties are not listed
The documentation at https://docs.gtk.org/ does not list child properties for container widgets.
Compare the gi-docgen output with the gtk-doc output. For example, the string tab-label
does not appear at all in the former page.
This omission makes a significant chunk of API surface invisible, which can be the difference between a developer finding the API they need and giving up frustrated. gi-docgen should list child properties much as gtk-doc does.