Various docs tweaks

Thanks @tomdyson for spotting!

docs/topics/images.rst

@@ -203,9 +203,9 @@ Output image format
 
 Wagtail may automatically change the format of some images when they are resized:
 
- - PNG and JPEG images always won't change format
+ - PNG and JPEG images don't change format
  - GIF images without animation are converted to PNGs
- - BMP images are always converted to PNGs
+ - BMP images are converted to PNGs
 
 It is also possible to override the output format on a per-tag basis by using the
 ``format`` filter after the resize rule.
@@ -237,8 +237,15 @@ quality:
     # Make low-quality but small images
     WAGTAILIMAGES_JPEG_QUALITY = 40
 
-Note that this won't affect any previously generated images. You may want to
+Note that this won't affect any previously generated images so you may want to
-delete all renditions so they can regenerate with the new setting.
+delete all renditions so they can regenerate with the new setting. This can be
+done from the Django shell:
+
+.. code-block:: python
+
+    # Replace this with your custom rendition model if you use one
+    >>> from wagtail.wagtailimages.models import Rendition
+    >>> Rendition.objects.all().delete()
 
 Changing per-tag
 ^^^^^^^^^^^^^^^^

wagtail/wagtailimages/models.py

@@ -417,8 +417,7 @@ class Filter(models.Model):
                 'original-format': original_format,
             }
             for operation in self.operations:
-                # Check that the operation can take the new "env" argument
+                # Check that the operation can take the "env" argument
-                # (added in Wagtail 1.5)
                 try:
                     inspect.getcallargs(operation.run, willow, image, env)
                     accepts_env = True

wagtail/wagtailimages/tests/test_image_operations.py

@@ -414,7 +414,7 @@ class TestFilter(TestCase):
         self.assertEqual(run_mock.call_count, 2)
 
     def test_runs_operations_without_env_argument(self):
-        # The "env" argument was added in Wagtial 1.5. This tests that
+        # The "env" argument was added in Wagtail 1.5. This tests that
         # image operations written for 1.4 will still work
 
         run_mock = Mock()