v4: Expand documentation for "Available variables"

Created by: tordans

I am wondering what the variables that are documented at http://v4-alpha.getbootstrap.com/getting-started/options/ do.

There are a few things that could be improved for the documentation:

• The $spacer is no longer specified in this part of the code but still documented there
• "various components" should be more specific on this page, like a linked list of componets that shows the examples for each of the component
• Also, are there component-specific variables as well that I could use to cherry pick "this component with rounded border, that without"?
• And on each component-documentation page, there should be an example of both settings with link to the global variables documentation page
• maybe add a link to from the source to the doc? Like so:

// Options
//
// Quickly modify global styling by enabling or disabling optional features.
// See http://v4-alpha.getbootstrap.com/getting-started/options/