Docs: parse {custom} and display correct values

[Maxhodges]

should list some acceptable values here

only |   | {custom} | A row can appear only for a specific device, or screen sizes.

i get error

ReferenceError: computer is not defined

on <Grid.Column only={computer}>

[layershifter]

You're trying to use computer as variable while it's a string value:

<Grid.Column only='computer'>

[layershifter]

However, our docs plugin should parse all custom values, but it will be not easy. I saw sometime ago on react-docgen and seems they allow to pass custom handlers. Help in this direction is very necessary.

We also need to convert union's to enum's there.