Rename 'Attributes' to 'Parameters' to fix documentation formatting

[binste]

With the addition of method-based attribute setting in #2795, Sphinx started generate the documentation differently for the encoding channels. The documentation for attributes is now the first sentence of the documentation which is defined by _PropertySetter:

This is much less informative then what is in the docstring of altair.X and which would be shown otherwise:

    Attributes
    ----------

    shorthand : string
        shorthand for field, aggregate, and type
    aggregate : :class:`Aggregate`
        Aggregation function for the field (e.g., ``"mean"``, ``"sum"``, ``"median"``,
        ``"min"``, ``"max"``, ``"count"`` ).

        **Default value:** ``undefined`` (None)

        **See also:** `aggregate <https://vega.github.io/vega-lite/docs/aggregate.html>`__
        documentation.
    axis : anyOf(:class:`Axis`, None)
        An object defining properties of axis's gridlines, ticks and labels. If ``null``,
        the axis for the encoding channel will be removed.

        **Default value:** If undefined, default `axis properties
        <https://vega.github.io/vega-lite/docs/axis.html>`__ are applied.

        **See also:** `axis <https://vega.github.io/vega-lite/docs/axis.html>`__
        documentation.

I tried to disable this behavior but didn't find a way. However, I'd suggest to tackle the root cause of this directly instead of trying to reconfigure Sphinx. Following the numpydoc style for docstrings (see also ExampleError on this page), what is called Attributes in the Altair class docstrings should, in my understanding, be called Parameters as we document what parameters can be passed to e.g. altair.X and not what attributes it has after instantiation. As all Parameters usually also appear as Attributes (with the exception of the ones which are now replaced by the property-based methods) this didn't matter much so far. I'd suggest to now introduce this distinction by replacing Attributes with Parameters in the docstrings:

    Parameters
    ----------

    shorthand : string
        shorthand for field, aggregate, and type

After this change, the documentation renders again how it's supposed to:

[mattijn]

Good find!

[joelostblom]

Another great addition, thank you!