clarify documentation of remove_dir errors

remove_dir will error if the path doesn't exist or isn't a directory.

It's useful to clarify that this is "remove dir or fail" not "remove dir
if it exists".

I don't think this belongs in the title. "Removes an existing, empty
directory" is strangely worded-- there's no such thing as a non-existing
directory.  Better to just say explicitly it will return an error.

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