tests/cpydiff: Document complex() parsing difference.

In #17384 it was decided that fixing this difference was not worth the code
size increase.  So document it instead.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/cpydiff/types_complex_parser.py

@@ -0,0 +1,14 @@
+"""
+categories: Types,complex
+description: MicroPython's complex() accepts certain incorrect values that CPython rejects
+cause: MicroPython is highly optimized for memory usage.
+workaround: Do not use non-standard complex literals as argument to complex()
+
+MicroPython's ``complex()`` function accepts literals that contain a space and
+no sign between the real and imaginary parts, and interprets it as a plus.
+"""
+
+try:
+    print(complex("1 1j"))
+except ValueError:
+    print("ValueError")