Issue #864: Add comment after addition of 'style.'

This was present earlier.
Weston added this, and it describes where it will appear.

includes/sanitizers/class-amp-iframe-sanitizer.php

@@ -81,7 +81,7 @@ public function sanitize() {
 			$this->did_convert_elements = true;
 
 			$new_attributes          = $this->set_layout( $new_attributes );
-			$new_attributes['style'] = 'max-width:100%';
+			$new_attributes['style'] = 'max-width:100%'; // AMP_Style_Sanitizer will move this to the amp-custom style.
 			if ( isset( $new_attributes['width'] ) && isset( $new_attributes['height'] ) ) {
 				$this->add_or_append_attribute( $new_attributes, 'class', 'amp-wp-enforced-sizes' );
 			}