Clarify R503 requires terminal returns only when another branch returns a value (#197)

.rules/python-return.md

@@ -43,14 +43,14 @@ Ensure all branches explicitly return a value if any branch does.
 
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-## R503 — Add an Explicit Return at the End
+## R503 — Add an Explicit Return at the End When a Function May Return a Value
 
 ```python
 # BAD:
 def func(x):
     if x > 0:
         return x
-    # no return (bad)
+    # missing terminal return (bad)
 
 
 # GOOD:
@@ -60,7 +60,17 @@ def func(x):
     return -1
 ```
 
-Don't rely on implicit `None`—always return something at the end.
+Don't rely on implicit `None` if the function may return a value elsewhere—always
+return something at the end.
+
+Functions whose only possible result is `None` do not need a final bare `return`:
+
+```python
+# GOOD:
+def func():
+    do_something()
+    # implicit None is fine here
+```
 
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