Arviz: Add input core dims to plot_hdi doc strings

[canyon289]

Describe the bug
input_core_dims doc string is missing even though its documented below in the examples in both az.hdi and az.plot_hdi

https://arviz-devs.github.io/arviz/generated/arviz.plot_hdi.html#arviz.plot_hdi
https://arviz-devs.github.io/arviz/generated/arviz.hdi.html#arviz.hdi

Suggested fix
Add to both documentation strings

[OriolAbril]

It's a keyword argument passed as **kwargs, it's not only input_core_dims that can be used but any of the arguments accepted by wrap_xarray_ufunc which is referenced and linked in the dosctring.

I think it would be better to extend the explanation in the example that uses it to make this clear.

[abhisht51]

Can you provide some example of what needs to be added here.

[OriolAbril]

hdi

in hdi doc, we could change the

If we want to calculate the HDI over specified dimension of dataset, we can pass input_core_dims by kwargs:

like with something like

By default, ``hdi`` is calculated over the ``chain`` and ``draw`` dimensions. We can use the 
``input_core_dims`` argument of :func:`~arviz.wrap_xarray_ufunc` to change this. In this example 
we calculate the HDI also over the ``school`` dimension:

and also change the code right below from az.hdi(data, input_core_dims = [["chain"]]) to

az.hdi(data, input_core_dims = [["chain", "draw", "school"]])

plot_hdi

Change

Precalculate HDI interval per chain and plot separately:

to

``plot_hdi`` can also be given precalculated values with the argument ``hdi_data``. This example 
shows how to use :func:`~arviz.hdi` to precalculate the values and pass these values to ``plot_hdi``. 
Similarly to an example in ``hdi`` we are using the ``input_core_dims`` argument of :func:`~arviz.wrap_xarray_ufunc` 
to manually define the dimensions over which to calculate the HDI.

What do you think @canyon289 ?

@abhisht51 if this were clear now, would you like to work on this issue?

[abhisht51]

I have understood what needs to be modified in the docs. I would be issuing a pull request soon!!

[abhisht51]

Hi,
I think I have made the necessary PR however I would like to point out that the "Hacktoberfest" label added by @percygautam does not counts as a valid one as hacktoberfest has made my PR ineligible. I believe the topic should have been "hacktoberfest" instead. I am new to this so I don't really know.

[abhisht51]

No worries!!