How to add a tooltip to a Form API field?
Solution is to use #attributes attribute to provide form element with HTML title attribute, like that:

'name' =&gt; array(
  '#title' =&gt; t('Name'),
  '#type' =&gt; 'textfield',
  '#size' =&gt; 30,
  '#maxlength' =&gt; 45,
  '#default_value' =&gt; $item-&gt;Name,
  // '#description' =&gt; t("Item's name, no longer than 45 characters."),
  '#attributes' =&gt; array(
    'title' =&gt; t("Item's name, no longer than 45 characters."),
  )
),