Document .npmrc registry/auth support in npm_translate_lock docstring

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.

Start with the npm_translate_lock docstring and read the linked npm documentation on project-specific .npmrc files. Check how the Bazel downloader config flag relates to the settings from .npmrc. Done means the docstring explains registry and authentication support and the relevant configuration validation.

Written by the indexing model from the issue text.