doc: Modules, Packages, and Instances docs should be clarified to not imply special pkg directory

[cueckoo]

Originally opened by @smorel-plenty in https://github.com/cuelang/cue/issues/753

In the section on import paths there's a mention of module: example.com/pkg. I'm assuming the pkg here is not referring to the pkg cue-managed external dependencies directory but could instead be something like module example.com/mypackage for better clarity when reading the docs?