gh-145300: Document bytearray.extend() behavior when mutating during iteration

Add a note to sequence.extend() documentation clarifying that bytearray
behaves differently from list and array.array when extending a sequence
with itself during iteration. bytearray captures the length at the start
and can at most double the original length, while list and array.array
pick up elements as they're added, potentially leading to unbounded growth.

Author: anand-official

Doc/library/stdtypes.rst

@@ -1328,6 +1328,17 @@ Mutable sequence types also support the following methods:
    For the most part, this is the same as writing
    ``seq[len(seq):len(seq)] = iterable``.
 
+      .. note::
+
+         When extending a sequence with itself (e.g., ``seq.extend(seq)``),
+         the behavior differs between sequence types if the sequence mutates
+         during iteration:
+
+         * :class:`list` and :class:`array.array` pick up elements as they
+           are added, potentially leading to unbounded growth.
+         * :class:`bytearray` captures the sequence length at the start,
+           limiting the extension to at most doubling the original length.
+
 .. method:: bytearray.insert(index, value, /)
             list.insert(index, value, /)
    :no-contents-entry: