Updated documentation for Contextual Filters in UI

I noticed that the help popup for Contextual Filters in the UI still uses the old Views language, referring to everything as 'arguments.' I've benefited greatly from the Views maintainers, and I wanted to try to contribute a bit where I can.

I thought this would be a simple search and replace, but I ended up moving quite a bit around so that the structure of the help document matches the structure of the actual Contextual Filters menu. I made relatively small style and language edits - mostly it's just about making sure that there is a descriptive entry for all of the options under contextual filters.

If this is useful, I'd be happy to go through and make any other similar changes that need to be made so that the documentation fits the awesome new UI.