Update styleguidist docs for Media.Plain (#3541)
Update styleguidist docs for Media.Plain (#3541)

Addresses part of https://github.com/nteract/nteract/issues/3342.
rgbkrk/nteract,rgbkrk/nteract,nteract/nteract,rgbkrk/nteract,nteract/nteract,nteract/nteract,nteract/nteract,rgbkrk/nteract,nteract/composition,nteract/composition,nteract/nteract,nteract/composition,rgbkrk/nteract