This way it's consistently used across all manual page without a need
for XSL templating.
Also, the entities file could in future possibly be used to template the
build-time configurables such as filesystem paths or bug tracker URL.
It's injected from the makefile, but not even used consistently or included in
the resulting render of manual page. Which is good, otherwise we'd have a
non-reproducible build with possible multilib conflicts if rendered around
midnight.
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.
