Create way of showing node/content-type specific help text

Use case:

On feedapi_inherit I would like to show helptext specific to content type configuration: I would like to tell the user that the feedapi_inherit processor has to be heavier than feedapi_item. I don't want this help text show up on nodes, because there I can't change the weight of a processor anyway.

We should come up with a mechanism that allows processor and parser module developers to define help text for the content type form, help text for the node form and help text for both.

Define

$form['help'] = array();
$form['help_node'] = array();
$form['help_content_type'] = array();

in hook_settings_form(). feedapi filters out help text that is not intended for the current form.

Before we can do this, we need to clean up _feedapi_invoke_settings_form() though - there are some duplicate lines in this one.