docs(ComponentProps): show func signatures

[levithomason]

This PR displays function signatures in the docs as part of #623.

These are generated from our doc block comments:

Button.js

/**
 * Called after user's click.
 * @param {SyntheticEvent} event - React's original SyntheticEvent.
 * @param {object} data - All props.
 */
onClick: PropTypes.func,

There is a known limitation with this PR. Only props with type func have their signatures displayed. If a prop uses a more complicated propType, it is parsed by react-docgen as type custom which we do not yet parse for a signature. We should consider adding @type {function} or @callback to callbacks with custom propType checkers, which we can parse to ensure their signatures are displayed.