Fix / remove warnings in Masonry documentation

[waywardmonkeys]

Separate from #389, the warnings when running cargo doc -p masonry need to be fixed.

Most of them are about things that no longer exist within masonry or have perhaps changed names, so it should be a pretty quick pass.

This is blocking re-enabling denying warnings within CI for xilem documentation and has already led to an issue in that regard with a broken link making its way into there.

[waywardmonkeys]

This is even worse with cargo clippy -- -W clippy::doc_markdown and that has even more regressions in xilem_core, xilem_web, and xilem.