Outdated documentation around 5.x API and authoring

[conobanegas]

There are some inconsistencies between the version 5.x API and the instructions at https://yeoman.io/authoring

For example: this.npmInstall() does not work and there is no such method in the API.

Sorry for my english, I speak in spanish.

[BirgitPohl]

In case of npmInstall()
Is there a workaround and can someone document it here?

Edit:
It is mentioned here: yeoman/generator#1593