Say why the form path prunes Type 3 fonts without a page

Annotation appearances draw through a page's resources, never through a
form's own, so the form path passes no page. The form's own patterns
and soft masks are still counted. Asked in review.

crates/redact/src/interp.rs

@@ -884,6 +884,8 @@ fn xobject(
                     out.error = Some(error);
                     return None;
                 }
+                // No page: annotation appearances draw through a page's resources, never through a
+                // form's. The form's own patterns and soft masks are still counted.
                 if let Err(error) = crate::type3::prune_type3_from_resources(doc, &mut cleaned_res, content, None) {
                     out.error = Some(error);
                     return None;