feat(explain): Deprecate & warn for `npm explain <path>`

[darcyclarke]

[ljharb]

This is distinct from npm explain <specifier>, yes?

Why deprecate it?

[darcyclarke]

@wraithgar I think you brought this up previously, can you add a quick summary?

[wraithgar]

I have no memory of this one

[darcyclarke]

@ljharb I think the confusion here was that a folder can be a valid package-spec. We're not removing the functionality here, we just update the documentation to clearly state npm explain supports a package-spec.

I'm going to close this since the work actually was completed by @wraithgar here: npm/cli#5048

Thanks for flagging.