Improve bytearray cast comment

h11/_readers.py

@@ -37,9 +37,6 @@
 __all__ = ["READERS"]
 
 header_field_re = re.compile(header_field.encode("ascii"))
-
-# Remember that this has to run in O(n) time -- so e.g. the bytearray cast is
-# critical.
 obs_fold_re = re.compile(rb"[ \t]+")
 
 
@@ -52,6 +49,7 @@ def _obsolete_line_fold(lines: Iterable[bytes]) -> Iterable[bytes]:
             if last is None:
                 raise LocalProtocolError("continuation line at start of headers")
             if not isinstance(last, bytearray):
+                # Cast to a mutable type, avoiding copy on append to ensure O(n) time
                 last = bytearray(last)
             last += b" "
             last += line[match.end() :]