libnm: add warning for bindings about broken functions for transferred GPtrArray

In commit 201c153e ('libnm: fix GObject Introspection annotations
for functions returning a GPtrArray') these annotations were changed
to fix Vala bindings. However, bindings may treat the transfer
annotation for GPtrArray differently, so depending on the binding
we either get a leak or a double free.

It's unclear how to fix that. For now, just add a warning to the
documentation to avoid it.

The following reproducer leads to a crash:

    #!/bin/python

    import gi

    gi.require_version("NM", "1.0")
    from gi.repository import NM

    def _pr(msg):
        NM.utils_print(0, msg + "\n")

    def process(nmc):
        for device in nmc.get_devices():
            cons = device.filter_connections(nmc.get_connections())
            _pr(
                "device %s (%s) has %s compatible connections"
                % (device.get_iface(), NM.Object.get_path(device), len(cons))
            )

    process(NM.Client.new())

See-also: https://gitlab.gnome.org/GNOME/gobject-introspection/-/issues/305