add a note on unsupported func+axis combination for frame_apply

Author: matiaslindgren

pandas/core/shared_docs.py

@@ -49,6 +49,8 @@
 for more details.
 
 A passed user-defined-function will be passed a Series for evaluation.
+
+If `func` defines an index relabeling, `axis` must be `0` or `index`.
 {examples}"""
 
 _shared_docs["compare"] = """