[2.1, doc] `machines create` CLI and API docs need power_parameters reference

When using the CLI on 2.1.3 for creating machines, and going by the help and API docs, there is no indication of power_parameters usage or requirements under "machine create." However, if omitted, it is announced as a requirement.

See also, "Create a new Machine" @ http://docs.maas.io/2.1/api.html

⟫ maas maas machines create hostname="test-machine" power_type="ipmi" mac_addresses="c0:ff:ee:00:00:00" architecture="amd64"

{"power_parameters": ["This field is required."]}

.

⟫ maas maas machines create --help
usage: /usr/bin/maas maas machines create [--help] [-d] [-k] [data [data ...]]

Create a new Machine.

This method accepts keyword arguments. Pass each argument as a
key-value pair with an equals sign between the key and the value:
key1=value1 key2=value key3=value3. Keyword arguments must come after
any positional arguments.

Adding a server to MAAS puts it on a path that will wipe its disks
and re-install its operating system, in the event that it PXE boots.
In anonymous enlistment (and when the enlistment is done by a
non-admin), the machine is held in the "New" state for approval by a
MAAS admin.

The minimum data required is:
architecture=<arch string> (e.g. "i386/generic")
mac_addresses=<value> (e.g. "aa:bb:cc:dd:ee:ff")

:param architecture: A string containing the architecture type of
    the machine. (For example, "i386", or "amd64".) To determine the
    supported architectures, use the boot-resources endpoint.
:type architecture: unicode

:param min_hwe_kernel: A string containing the minimum kernel version
    allowed to be ran on this machine.
:type min_hwe_kernel: unicode

:param subarchitecture: A string containing the subarchitecture type
    of the machine. (For example, "generic" or "hwe-t".) To determine
    the supported subarchitectures, use the boot-resources endpoint.
:type subarchitecture: unicode

:param mac_addresses: One or more MAC addresses for the machine. To
    specify more than one MAC address, the parameter must be specified
    twice. (such as "machines new mac_addresses=01:02:03:04:05:06
    mac_addresses=02:03:04:05:06:07")
:type mac_addresses: unicode

:param hostname: A hostname. If not given, one will be generated.
:type hostname: unicode

:param domain: The domain of the machine. If not given the default
    domain is used.
:type domain: unicode

:param power_type: A power management type, if applicable (e.g.
    "virsh", "ipmi").
:type power_type: unicode

Common command-line options:
    --help, -h
        Show this help message and exit.
    -d, --debug
        Display more information about API responses.
    -k, --insecure
        Disable SSL certificate check