fix sphinx issue and add note to docs

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: MyreMylar

buildconfig/stubs/pygame/transform.pyi

@@ -480,5 +480,7 @@ def pixelate(
 
     :returns: A new surface that's been pixelated.
 
-    ..versionadded:: 2.5.6
+    .. versionadded:: 2.5.6
+    .. note::
+        ``pixel_size`` must be >= 1. A ``ValueError`` is raised otherwise.
     """