Change Subscription Block Form; remove text - "The subscriber's email address."; add placeholder

Thank you @robcarr for posting this! It works!

For anyone also looking to change the Submit button text and adding a title attribute to it:

<?php
/**
 * Implements hook_form_alter().
 */
function mytheme_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'simplenews_subscriptions_block_simplenews_mycategory') {
    $form['actions']['submit']['#value'] = t('Notify me');
    $form['actions']['submit']['#attributes']['title'] = t('Receive email reminders about new articles published on this site');
    $form['mail']['widget'][0]['value']['#attributes']['placeholder'] = 'myemail@example.com';
  }
}
?>