Docs linking see elements with non-documented symbols to root

[daveaglick]

The problem happens when there's a <see cref="..."/> element in the docs and the cref points to a symbol from the standard library or elsewhere that we might have imported but didn't specify as documentation output.

See https://github.com/Wyamio/Wyam/blob/e674d45e96867f3cd3764bf344f4de70a3f3a85f/src/extensions/Wyam.CodeAnalysis/Analysis/XmlDocumentationParser.cs#L716 - presumably the link is populated but is either and empty string or something like a / or ..