update documentation to say that `id` can be a dictionary as well as a string

[nopria]

At https://dash.plotly.com/dash-core-components/dropdown it says that the id is a string, while as far as I understand it can be a dictionary too.

[alexcjohnson]

Thanks for raising this @nopria. In fact this is the case for all components, not just dropdown and not just DCC. We should really make this a special case in the component generator, because id as dict is handled at the dash-renderer level and converted to a string for components to render into the DOM. That means as far as component authors are concerned, id really is a string, but as far as app developers are concerned it can be either a string or a dict.