Docs: Drop entities, switch away from sgml mode

Docs: Drop entities, switch away from sgml mode

Since all element markup is now gone from the doc comments,
we can turn off the gtk-doc sgml mode, which means that from
now on, docbook markup is no longer allowed in doc comments.

To make this possible, we have to replace all remaining
entities in doc comments by their replacement text, & -> &
and so on.