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.

[PoignardAzur]

This should be fixed by #815. Are there still errors?

[DJMcNab]

This issue also tracks re-enabling checking for these issues in CI. Is that resolved?

[PoignardAzur]

Well I didn't change the CI scripts after fixing the doc errors, so probably not.

[DJMcNab]

It looks like https://github.com/linebender/xilem/pull/815/files#diff-27b19fb95b25f32e39ea044b6ab68ee4a84d37944078b20ea52985fcca779e7aL112-L115 should have addressed this, I think.

The only thing that's still wrong is

xilem/masonry/src/app/tracing_backend.rs

Line 4 in d77b31b

#![warn(rustdoc::broken_intra_doc_links, clippy::doc_markdown, missing_docs)]

which is redundant I think. It looks like these were being suppressed by allows rather than anything in the CI script.

[waywardmonkeys]

@DJMcNab I'd removed that locally yesterday but didn't get to submitting the PR. Done now as #855.