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.
