Added documentation to the FormBuilder class

This should help clarify issue #10115. The documentation attempts to explain what the ``FormBuilder`` class does. 

I've also changed the ``field_helpers`` attribute to an explicit list of methods. The reason I did this was because the old way of defining the ``field_helpers`` called ``FormHelper.instance_methods`` and subtracted away some methods. It is hard to figure out what methods these are without running the code, so I think that this change so make things more readable and understandable. Thoughts?