Add note about accessor instance creation

[djhoese]

This is an attempt at clarifying accessor docs to help with the confusion I had in #3205. I'm not sure how many other xarray doc pages use .. note:: but it seemed useable here. I'm not sure this is worth the explanation or if it is worded the best, but thought this could start the discussion.

• Closes Accessors are recreated on every access #3205
• Tests added
• Passes black . && mypy . && flake8
• Fully documented, including whats-new.rst for all changes and api.rst for new API

[shoyer]

Thanks @djhoese, this is definitely worth noting!