[Docs]: Improve discoverability of Multiselect.Filterable component

[vpicone]

Summary

Improve documentation for the Filterable version of Multiselect

Justification

The Multiselect has some pretty gnarley performance issues when lists are large. The Filterable multiselect component has the same performance issues, but it makes for a better experience with large lists.

The only way you'd find it is digging through props in the storybook knobs. Because this is a totally different component, it should be a new story rather than tucked into the props.

Desired UX and success metrics

Create a new story for Multiselect.Filterable