tests/cpydiff: Clarify f-string diffs regarding concatenation.

Concatenation of any literals (including f-strings) should be avoided.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/cpydiff/core_fstring_concat.py

@@ -1,12 +1,13 @@
 """
 categories: Core
-description: f-strings don't support concatenation with adjacent literals if the adjacent literals contain braces
+description: f-strings don't support concatenation with adjacent literals if the adjacent literals contain braces or are f-strings
 cause: MicroPython is optimised for code space.
-workaround: Use the + operator between literal strings when either is an f-string
+workaround: Use the + operator between literal strings when either or both are f-strings
 """
 
-x = 1
+x, y = 1, 2
-print("aa" f"{x}")
+print("aa" f"{x}")  # works
-print(f"{x}" "ab")
+print(f"{x}" "ab")  # works
-print("a{}a" f"{x}")
+print("a{}a" f"{x}")  # fails
-print(f"{x}" "a{}b")
+print(f"{x}" "a{}b")  # fails
+print(f"{x}" f"{y}")  # fails