Documentation: selectizeInput doesn't explain what selectize.js means

[daattali]

The argument selectize says "Whether to use selectize.js or not". Most shiny users probably don't know that selectize.js is a javascript dropdown library, and that argument is a mystery to them.

Similarly, I've seen many intermediate shiny users being completely unaware of the difference between selectInput() and selectizeInput(). It would be good to explain that difference.