Clean up advection docs

[dopplershift]

Right now the documentation for parameters for advection looks like:

    Parameters
    ----------
    scalar : `pint.Quantity` or `xarray.DataArray`
        Array (with N-dimensions) with the quantity to be advected. Use `xarray.DataArray` to
        have dimension ordering automatically determined, otherwise, use default
        [..., Z, Y, X] ordering or specify \*_dim keyword arguments.
    u, v, w : `pint.Quantity` or `xarray.DataArray` or None
        N-dimensional arrays with units of velocity representing the flow, with a component of
        the wind in each dimension. For 1D advection, use 1 positional argument (with `dx` for
        grid spacing and `x_dim` to specify axis if not the default of -1) or use 1 applicable
        keyword argument (u, v, or w) for proper physical dimension (with corresponding `d\*`
        for grid spacing and `\*_dim` to specify axis). For 2D/horizontal advection, use 2
        positional arguments in order for u and v winds respectively (with `dx` and `dy` for
        grid spacings and `x_dim` and `y_dim` keyword arguments to specify axes), or specify u
        and v as keyword arguments (grid spacings and axes likewise). For 3D advection,
        likewise use 3 positional arguments in order for u, v, and w winds respectively or
        specify u, v, and w as keyword arguments (either way, with `dx`, `dy`, `dz` for grid
        spacings and `x_dim`, `y_dim`, and `vertical_dim` for axes).
    dx, dy, dz: `pint.Quantity` or None, optional
        Grid spacing in applicable dimension(s). If using arrays, each array should have one
        item less than the size of `scalar` along the applicable axis. If `scalar` is an
        `xarray.DataArray`, these are automatically determined from its coordinates, and are
        therefore optional. Required if `scalar` is a `pint.Quantity`. These are keyword-only
        arguments.
    x_dim, y_dim, vertical_dim: int or None, optional
        Axis number in applicable dimension(s). Defaults to -1, -2, and -3 respectively for
        (..., Z, Y, X) dimension ordering. If `scalar` is an `xarray.DataArray`, these are
        automatically determined from its coordinates. These are keyword-only arguments.

It's ok, but there are a lot of details in the parameters that might be better done in the block above generally describing the function. The problem, though, is what happens in the web docs:

That wall of repeated chunks of indecipherable text just isn't useful to anyone.

When fixing this we should also add our mathematical definition of advection to the Notes section.