clean-unref: documentation & feat suggestion (recursivity)

Hi!

About documentation

cargo cache -h
...
    clean-unref    remove crates that are not referenced in a Cargo.toml from the cache

When I read this I guess that it looks around for Cargo.toml to list all the dependencies and remove data in the cache that are not used by any of them. In the README is mentioned:

• purge cache entries not unused to build a specified crate (cargo cache clean-unref)

Then I understand that it does this only for the crate I am in. When I run the command, from my home directory, I get:

% cargo cache clean-unref
...
Failed to Cargo.toml manifest in /home/***** or downwards.

So I think the help description for this subcommand could be improved (for instance copying the README)

The suggestion

This feature as I currently understand it removes unused crates (but not binaries) that are not listed in the Cargo.toml of the crate I am currently in, therefore ignoring all the other crates. I think it can be good for CI use, but for personal use, I think that it could be interesting to have a command like clean-unref-user which (Rust-like pseudocode):

• let used = Iterates over all Cargo.toml in user directory recursively (can take some time)
• .map(|file| file. get the list of the dependencies ()).flatten();
• Get the list of the repositories
• for dependency in used { remove dependency from the list of repositories (skip if already removed or missing) }
• The remaining repositories are not used so can be deleted.

Start with the cargo cache -h output, the README description of clean-unref, and the command's behavior from a crate directory versus the home directory. Decide whether the work is limited to clarifying documentation or includes recursive discovery of user Cargo.toml files. Done requires a defined scope and help text that accurately describes the supported behavior.