Add a quick description of the geom_getxml(3), geom_xml2tree(3),
geom_gettree(3) and geom_deletetree(3) functions provided by libgeom and are
not documented in libgeom(3).

Reviewed by: mav, bjk, allanjude
Approved by: allanjude
Differential Revision: https://reviews.freebsd.org/D12679