cy.task is missing the "plugins API docs"

[brian-mann]

cy.task is hardly documented for the background JS process.

Nowhere does it mention that returning a promise is awaited. Even the example utilizes fsExtra without ever even requiring it in the pluginsFile.

Can we please document cy.task in the list of plugin events and please go into more detail about how / what you can do with it.

Here's a question for reference: cypress-io/cypress#2152

[brian-mann]

We should also have an example of async/await in the background process.