Display defaults and requirements for route path variables.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ---
| Refs tickets  | ---
| License       | MIT
| Doc PR        | ---

It should eventually replaced by a [BS Popover](http://getbootstrap.com/javascript/#popovers) at some point.