remove unclear comment and add docstring to explain why

--HG--
extra : transplant_source : %C4Y%8Fo%10QD%23%A6%FB%B6%AF%3B%D6%C9%A9%09%C4%0D%A8

Cython/Compiler/ParseTreeTransforms.py

@@ -2530,6 +2530,9 @@ class GilCheck(VisitorTransform):
 
 
 class TransformBuiltinMethods(EnvTransform):
+    """
+    Replace Cython's own cython.* builtins by the corresponding tree nodes.
+    """
 
     def visit_SingleAssignmentNode(self, node):
         if node.declaration_only:

Cython/Compiler/Pipeline.py

@@ -185,7 +185,7 @@ def create_pipeline(context, mode, exclude_classes=()):
         AutoTestDictTransform(context),
         EmbedSignature(context),
         EarlyReplaceBuiltinCalls(context),  ## Necessary?
-        TransformBuiltinMethods(context),  ## Necessary?
+        TransformBuiltinMethods(context),
         MarkParallelAssignments(context),
         ControlFlowAnalysis(context),
         RemoveUnreachableCode(context),