DllPlugin: When processing multiple chunks, does the path option must have a string template?

[cwj0417]

When generating Dll files, It seems like every chunk should emit a manifest file.

If we not config [name] (or [hash]) placeholder in path option, all chunks was written in a single file and leads to undesirable result.

If what I described above is correct, I considered we could add some explanations to document and print warning on console. and can I send a PR improve this?

To reproduce: change path option in examples/dll/webpack.config.js to path.join(__dirname, "dist", "manifest.json"), and run node build.js for several times, manifest.json is occasionally written to an illegal json.

(I don' t think this is a bug, so I raise this issue with "other" template)