Document providing list of products to `dc.load`

[robbibt]

Datacube supports loading data from multiple products at once by providing a list of products to dc.load, e.g.:

ds = dc.load(
    product=["ga_s2am_fmc", "ga_s2bm_fmc", "ga_s2cm_fmc"],
    **query)

However, this functionality is not mentioned in the documentation here: https://opendatacube.readthedocs.io/en/latest/api/indexed-data/generate/datacube.Datacube.load.html

We should update the product ([str] | [None]) documention to mention that lists of products can be provided, with the caveat that this is best applied only to products that share the same band names.