[Documentation] What are mixins?

[ericxinzhang]

I am learning Yeoman and going to write my own generator. I see actions/mixins in the API doc but they are never mentioned in the Getting Started guides for Using Yeoman/Creating a Generator.

The mixins look like the member fields in the generator to me but there are inconsistencies. Examples are:

• The mixin user is used as a member field of the generator instance here.
• The installDependencies is called directly on this here.

Could you please update the documentation/guides to shed some lights on the actions/mixins?