Hint about lacking support for links to operators

[MichaelChirico]

Updating from {roxygen2} 7.3.2 to 7.3.3 generated a lot of logs like:

✖ integer64.R:171: @returns Could not resolve link to topic "\\%/\\%" in the dependencies or base packages.
ℹ If you haven't documented "\\%/\\%" yet, or just changed its name, this is normal. Once "\\%/\\%" is documented, this warning goes away.
ℹ Make sure that the name of the topic is spelled correctly.
ℹ Always list the linked package as a dependency.
ℹ Alternatively, you can fully qualify the link with a package name.

The current messaging is misleading / extra hard to parse due to the double-escaped \. It took some sleuthing to find that this is not supported: #550. (I had checked the Indexing and cross-references vignette, not [(R)Markdown support]((R)Markdown support)).

Looks like this entails new shapshots -- can generate if this roughly looks good.