Especially for "nm-settings-docs-nmcli.xml", the first XML to merge is
"clients/cli/generate-docs-nm-settings-nmcli.xml". That file is
generated with the meta data from nmcli, and it contains all the
properties that are supported. Properties from other XML files,
that are passed as additional arguments should not be merged.
In most cases, there is no difference. It only matters for
"ipv6.dad-timeout" and "user.data". For example, "ipv6.dad-timeout"
is supported by GObject (part of "libnm/nm-settings-docs-gir.xml"),
but not by nmcli. Don't include it in the manual.
This also drops the now empty settings "dummy", "user", and "generic".
