Allow descriptions for oneOfType values

[cangoektas]

Hey!

Hope you don't mind me putting this PR together without a clear consent for this feature. I just had a lot of fun diving into this 😄

Changes

Allows items of oneOfType to have descriptions as well, similar to properties of shape. Fixes #300.

Example

{
  foo: PropTypes.shape({
    /** This was possible previously */
    a: PropTypes.string
  }),
  bar: PropTypes.oneOfType([
    /** Now this is possible, too */
    PropTypes.string,
    PropTypes.number
  ])
}

[danez]

Thank you very much for this. This looks great. Feel free to create more PRs :)

[cangoektas]

@danez nice, thanks a lot! Is there also a chance to get this released soon? 😄