These remaining DocBook tags are mostly harmless; they are passed as
HTML5 tags to the browser, which effectively treats them as just
<span> elements, so all this mistake is doing is dropping some
styling.

This lack of of styling can be seen at:

 - https://docs.gtk.org/gio/enum.DriveStartStopType.html "ATA SECURITY
   UNLOCK DEVICE" is not indicated to be a quote or anything.

 - https://docs.gtk.org/gio/struct.UnixMountEntry.html "/media/cdrom"
   is not rendered in monospace.

 - https://docs.gtk.org/gio/struct.UnixMountPoint.html "/dev" is not
   rendered in monospace.

 - https://docs.gtk.org/glib/type_func.Thread.init.html the notes are
   not indicated to be anything other than regular paragraphs.

Fix that.