It's hard to understand what "tools/generate-docs-nm-settings-docs-merge.py"
does. Add dbg() statements that are all NOP by default. But the user can
easily patch the code to print what is happening. This is only for
debugging the script.
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".