Extensions pick up documentation from other elements

[pcantrell]

SourceKitten pulls documentation from other elements for extensions. Here's an example where SourceKitten ignores an extension’s doc comment and instead copies the extended class’s doc comment.

That example is innocuous enough, almost reasonable, but I noticed this problem in a more complex project where a public extension was pulling a :nodoc: from who knows where. Both the extension and the class it extended were public, but Jazzy ignored the extension because of this spurious SourceKitten behavior. (The extension that got the mystery :nodoc: is here. I worked around it locally with a cludge in jazzy to ignore nodoc whenever it appears on an extension.)