Title on the spam honeypot field

Per accessibility guidelines, this input should have a title or a label
to indicate what it's for. Fixes #3260

modules/sharedaddy/sharing-sources.php

@@ -515,7 +515,8 @@ public function display_footer() {
 				<input type="email" name="source_email" id="source_email" value="" />
 
 			<?php endif; ?>
-			<input type="text" id="jetpack-source_f_name" name="source_f_name" class="input" value="" size="25" autocomplete="off" />
+			<input type="text" id="jetpack-source_f_name" name="source_f_name" class="input" value="" size="25" autocomplete="off" title="This field is for validation and should not be changed" />
+			<script>jQuery( document ).ready( function(){ document.getElementById('jetpack-source_f_name').value = '' });</script>
 			<?php
 				/**
 				 * Fires when the Email sharing dialog is loaded.