Refactor: Add some doc string to pin-form-referer

Also change the input type to url.

pinry/templates/includes/pin_form.html

@@ -19,7 +19,10 @@
                             </div>
                             <div class="form-group">
                                 <label for="pin-form-referer">Referer</label>
-                                <input name="pin-form-referer" id="pin-form-referer" class="form-control" type="text"/>
+                                <input
+                                    name="pin-form-referer" id="pin-form-referer" class="form-control" type="url"
+                                    placeholder="Referer URL to the page which includes your pin"
+                                />
                             </div>
                             <div class="form-group">
                                 <label for="pin-form-description">Description</label>