Documentation: mention required=True for options

This is never mentioned as a possibility for click.option anywhere in the docs as far as I can tell (or at least it should be more prominent, maybe a section). (it talks about it in click.argument, but its unclear that it exists for both)

Someone just came to me saying, "we have to stop using click, i checked everywhere in their documentation, they don't support required options, which is important!"

Thanks :)