docs(angular.bootstrap): clarify modules parameter

It was not clear what you could pass to specify modules to load in the
`module` parameter of this function. The `modules` parameter takes an
array.

The main case is to provide a String, which is the name of a "predefined"
angular module.
The side cases are to provide a Function (or an annotated function in the
form of an Array), which will be invoked by the injector as a run block.

It is not possible to "define" new modules via this parameter.

Closes #3692