DataArray.mean docstring is not filled in completely

[shoyer]

as reported by @chrisroat:

In [9]: help(normed.mean)
Help on method mean in module xarray.core.common:

mean(self, dim=None, axis=None, skipna=None, keep_attrs=False, **kwargs) method of xarray.core.dataarray.DataArray instance
    Reduce this DataArray's data by applying `mean` along some
    dimension(s).

    Parameters
    ----------
    dim : str or sequence of str, optional
        Dimension(s) over which to apply `{name}`.
    axis : int or sequence of int, optional
        Axis(es) over which to apply `{name}`. Only one of the 'dim'
        and 'axis' arguments can be supplied. If neither are supplied, then
        `{name}` is calculated over axes.
    skipna : bool, optional
        If True, skip missing values (as marked by NaN). By default, only
        skips missing values for float dtypes; other dtypes either do not
        have a sentinel missing value (int) or skipna=True has not been
        implemented (object, datetime64 or timedelta64).
    keep_attrs : bool, optional
        If True, the attributes (`attrs`) will be copied from the original
        object to the new one.  If False (default), the new object will be
        returned without attributes.
    **kwargs : dict
        Additional keyword arguments passed on to `mean`.

    Returns
    -------
    reduced : DataArray
        New DataArray object with `mean` applied to its data and the
        indicated dimension(s) removed.

We probably should fix #755 at the same time.