Retrieves a form from a builder function, passes it on for
processing, and renders the form or redirects to its destination
as appropriate. In multi-step form scenarios, it handles properly
processing the values using the previous step's form…
The #pre_render of a form allows us to make changes AFTER validation (unlike
hook_form_alter()), but BEFORE the form has actually been displayed. We use
it to control which form elements are shown, which are hidden, and which values
to set based on…