docs($q): explain what the $q service does in description
docs($q): explain what the $q service does in description

Explain what the $q service does in description, instead of origin document.

The original explanation was less accessible to people new to promises and JS in general.

Closes #10056

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js