How to put description on top of the input box, for a new field added to comment form?
You need to add a description field to your array:

$form['paragraph_id'] = array(
    '#type' =&gt; 'textfield',
    '#title' =&gt; t('Your Title'),
    '#description' =&gt; t('The paragraph you would like to put a comment on (enter 0 if you want to put the comment on the entire article'),
    ...
);


This way the explanation will show up under the field.