How to add collapsible help text to a form?
What about putting it into a fieldset ? 

$form['fieldset'] = array(
 '#title' =&gt; 'title',
 '#type' =&gt; 'fieldset',
 '#collapsible' =&gt; TRUE,
 '#description' =&gt; 'desc',
);

$form['fieldset']['some_text'] = array(
  '#markup' =&gt; '&lt;p&gt;Here is some very useful text.&lt;/p&gt;',
  '#weight' =&gt; -1,
);