Document .npmrc registry/auth support in npm_translate_lock docstring

[alexeagle]

https://docs.npmjs.com/using-private-packages-in-a-ci-cd-workflow#create-and-check-in-a-project-specific-npmrc-file

In practice we probably just have to validate that the Bazel downloader config flag is set, and the downloader config has mirrored the settings from .npmrc.

[pedrobarco]

I think this has been fixed with #475

[alexeagle]

I think we're missing a documentation update to consider it done

[gregmagolan]

Basic auth support has landed in #623. I think we should just add more color to the npm_translate_lock docstring for the npmrc setting and we can close this one.