Add comment about prefix count

src/black/strings.py

@@ -136,8 +136,9 @@ def normalize_string_prefix(s: str, remove_u_prefix: bool = False) -> str:
     match = re.match(r"^([" + STRING_PREFIX_CHARS + r"]*)(.*)$", s, re.DOTALL)
     assert match is not None, f"failed to match string {s!r}"
     orig_prefix = match.group(1)
-
     new_prefix = orig_prefix.replace("F", "f").replace("B", "b").replace("U", "u")
+
+    # Python syntax guarantees max 2 prefixes and that one of them is "r"
     if len(new_prefix) == 2 and "r" != new_prefix[0].lower():
         new_prefix = new_prefix[::-1]
     if remove_u_prefix: