<description> is now an XML element, no longer an attribute. Fix the
style sheets.
Fixes: 89abede3df ('docs: rework generating property infos to use <description> element')
Since libnm-core secret-flags properties are now enum-typed rather
than just being uints, we can now actually recognize them when
generating docs, rather than just assuming that every property whose
name ends in '-flags', but isn't in NMSettingDcb, is a secret-flags
property.
