Update documentation around hook_element_info() and drupal_render() to not say #type is limited to the Form API

The docs for hook_element_info() have barely been touched since 2008.

These days #type is just a generic way to have defaults merged into renderable arrays and isn't limited to the Form API.

See #type 'markup', 'page', 'html_tag' for examples of non-form #types.

I think mostly we just need to replace "Form API" everywhere in the element doces with "Render API" and we should be fine.