SI-8591 Scaladoc: Problem with scrolling under Safari on iOS

This PR resolves some 'small but snippy' problem with scrolling scaladoc contents under Safari on iOS.

To ease reviewing my changes, I've published two versions of generated documentation:

1. Version without fix (you can't scroll the right frame): http://scaladoc-permalinks.herokuapp.com/index.html#scalaz.CoyonedaInstances0@finalize():Unit

2. Version with CSS fix: http://scaladoc-ios.herokuapp.com/index.html#scalaz.CoyonedaInstances0@finalize():Unit

My branch came out from 2.12.x but I can rebase it if needed.

I've signed Scala CLA.

OT: I regret that I didn't fix it before I finished reading 'scala puzzlers' - exploring scaladocs would be then much more comfortable. Maybe it's worth to publish this fix before that book will be published? There is not so much to review..