Missing documentation on how the `extensions` option is used.

[call-a3]

I experience some unexpected behaviour when I pass the extensions option and can't find any information on how this option is used so that I can debug my usage of this package.

[ljharb]

See the third bullet point under https://github.com/substack/node-resolve#resolveid-opts-cb, and below that, the default value of extensions: [ '.js' ]