Document `userAgent` option
Document `userAgent` option

Fixes #170.

sindresorhus/pageres,portgasd666/pageres,RyanTech/pageres,JuanitoFatas/pageres,RyanTech/pageres,elgubenis/pageres,portgasd666/pageres,elgubenis/pageres,sindresorhus/pageres,JuanitoFatas/pageres,Ahmed-Badawy/pageres,Ahmed-Badawy/pageres,sindresorhus/pageres