implement help_text on chooser blocks

wagtail/wagtailcore/blocks/field_block.py

@@ -306,14 +306,17 @@ class RawHTMLBlock(FieldBlock):
 
 
 class ChooserBlock(FieldBlock):
-    def __init__(self, required=True, **kwargs):
+    def __init__(self, required=True, help_text=None, **kwargs):
         self.required = required
+        self.help_text = help_text
         super(ChooserBlock, self).__init__(**kwargs)
 
     """Abstract superclass for fields that implement a chooser interface (page, image, snippet etc)"""
     @cached_property
     def field(self):
-        return forms.ModelChoiceField(queryset=self.target_model.objects.all(), widget=self.widget, required=self.required)
+        return forms.ModelChoiceField(
+            queryset=self.target_model.objects.all(), widget=self.widget, required=self.required,
+            help_text=self.help_text)
 
     def to_python(self, value):
         # the incoming serialised value should be None or an ID

wagtail/wagtailcore/tests/test_blocks.py

@@ -1218,7 +1218,7 @@ class TestPageChooserBlock(TestCase):
         self.assertEqual(block.to_python(None), None)
 
     def test_form_render(self):
-        block = blocks.PageChooserBlock()
+        block = blocks.PageChooserBlock(help_text="pick a page, any page")
 
         empty_form_html = block.render_form(None, 'page')
         self.assertIn('<input id="page" name="page" placeholder="" type="hidden" />', empty_form_html)
@@ -1227,6 +1227,7 @@ class TestPageChooserBlock(TestCase):
         christmas_form_html = block.render_form(christmas_page, 'page')
         expected_html = '<input id="page" name="page" placeholder="" type="hidden" value="%d" />' % christmas_page.id
         self.assertIn(expected_html, christmas_form_html)
+        self.assertIn("pick a page, any page", christmas_form_html)
 
     def test_form_response(self):
         block = blocks.PageChooserBlock()