Commit b2a0738765 ('man: improve manual page for nm-online') removed
the explanation of how may-fail can be used to wait for a specific
address family during boot. I found that part useful. Add it again,
adapting it to the new behavior introduced by 1e5206414a ('device:
don't delay startup complete for pending-actions "autoconf", "dhcp4"
and "dhcp6"').
https://bugzilla.redhat.com/show_bug.cgi?id=1825666
(cherry picked from commit 25583de20b)
This improves the HTML rendering.
But it also causes a lot of non-resolvable linkends warning when rendering a
separate manual pages into roff/mman. The messages are harmless, but still
a bit ugly.
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.
