Documentation: clarify how to use plugins in selectizeInput()

[daattali]

The blog post for shiny 1.8.0 release had a section about selectizeInput's upgrade and plugins. It shows an example of using options = list(plugins = "remove_button"). However, by just looking at the documentation for selectizeInput(), it would be very difficult to know that plugins are available and how to use them. The options argument docs simply refer to https://selectize.dev/docs/usage for possible options, but has no examples.

Because of extensive prior experience I do know how to read selectize's documentation, and how to translate that to shiny's selectizeInput. But for anyone who isn't already familiar with it, I would argue it's almost impossible to realize that they can use plugins and how to do so (without Googling). It's also very hard for them to know how to use other options. I recommend adding examples of using options in the docs.