docs: Fix mistakes from the GTK-Doc to GI-DocGen conversion
I am working on an alternative implementation of GI-DocGen's markdown parser, and testing it against gi-docgen. In this testing, I have come across a few places where the conversion from the markdownish/DocBook markup of GTK-Doc to the markdown/HTML markup of GI-DocGen isn't quite right or complete.
This MR fixes those conversion issues. There are more details in each commit message.