The docs for the documentation field say

If no URL is specified in the manifest file, crates.io will automatically link your crate to the corresponding docs.rs page.

This sounds as if setting no URL is equivalent to setting the docs.rs URL. That is however not the case: in the crates.io search result list, the "Documentation" link will only show up if documentation is explicitly set in the toml file.

I am not sure if this is a bug in the cargo docs or the crates.io frontend, but it is certainly surprising.