Move the settings/plugins doc generation from libnm-util to
libnm-core, since libnm-util isn't being updated for all new
properties.

With this commit, the keyfile and ifcfg-rh documentation is basically
unchanged, except that deprecated properties are now gone, and new
properties have been added, and the sections are in a different order.
(generate-plugin-docs.pl just outputs the settings in Makefile order,
and they were unsorted in libnm-util, but are sorted in libnm-core).

The settings documentation used for nm-settings.5, the D-Bus API docs,
and the nmcli help is changed a bit more at this point, and mostly for
the worse, since the libnm-core setting properties don't match up with
the D-Bus API as well as the libnm-util ones do. To be fixed...

(I also removed the "plugins docs" line in each plugin docs comment
block while moving them, since those blocks will be used for more than
just plugins soon, and it's sort of obvious anyway.)