Slider: Use string in event callback

[interactivellama]

Additional description

onChange and onInput now pass in event, { value: [string] }. This is a type change from @DanFerro great addition to the library. I've talked with him, and we agreed that it should be a string. This is not a breaking change, since the Slider was never released in a version.

Build docs is also run with updated prop comments.

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Pull Request Review checklist (do not remove)

• Review the appropriate Storybook stories. Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at snapshot strings.
• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
• Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
• Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
• While the contributor's branch is checked out, run npm run local-update within locally cloned site repo to confirm the site will function correctly at the next release.