Form Field description prop should accept a React component

[nkohari]

On Form Fields, the description prop (which shows the "helper text" beneath the field) must be a string. For example:

const MyComponent = props => (
  <Form>
    <Form.TextInput label="Form label" description="test" />
  </Form>
);

Instead, we should expand this to also allow a React component to be passed. For example:

const MyComponent = props => {
  const description = <Paragraph>Here is some <strong>rich-formatted</strong> helper text.</Paragraph>;
  return (
    <Form>
      <Form.TextInput label="Form label" description={description} />
    </Form>
  );
};

If the description value is a string, it should just be rendered as plain text.

[nkohari]

Right now, if the user supplies a string, we're doing replacement of backticks with <Code> components, similar to Markdown. Do we want to keep this behavior?

On one hand it might be nice to provide a simple shortcut, but I worry that having two alternatives might be a bit confusing.