glib: Remove unneeded POPPLER_STRUCTURE_ELEMENT_UNKNOWN

The enum value POPPLER_STRUCTURE_ELEMENT_UNKNOWN because Poppler does
not add invalid StructElements ("invalid" being nodes of type
StructElement::Unknown) to the Tagged-PDF structure tree. That means
that poppler-glib does not need to expose it in the API. An assertion
is left to aid in finding issues when using debug builds.

https://bugs.freedesktop.org/show_bug.cgi?id=75541
glib: Remove poppler_text_span_is_link() in header

The method poppler_text_span_is_link() does not exist, and must
be removed from the API header.

https://bugs.freedesktop.org/show_bug.cgi?id=75613
glib: Fixes in the API reference documentation

- Remove the references to non-existent methods for PopplerStructureElement,
  and point to PopplerStructureElementIter instead to point out how to
  obtain the structure tree.
- Remove a non-existent include in poppler-docs.sgml
- Add POPPLER_TYPE_TEXT_SPAN to the list of private symbols.

https://bugs.freedesktop.org/show_bug.cgi?id=75615