docs: Update ShareableList documentation for fixed trailing null bug

The bug where ShareableList stripped trailing null bytes has been fixed
in Python 3.15. Update documentation to:
- Note the fix with versionchanged directive
- Update doctest to show correct behavior (nulls preserved)
- Clarify workaround is only needed for Python 3.14 and earlier
- Reference both original issue #106939 and fix issue #145261

Fixes failing doctest in CI where expected output showed old buggy
behavior instead of corrected behavior.

Author: stefanzetzsche

Doc/library/multiprocessing.shared_memory.rst

@@ -313,26 +313,33 @@ finishes execution.
 
    .. note::
 
-      A known issue exists for :class:`bytes` and :class:`str` values.
-      If they end with ``\x00`` nul bytes or characters, those may be
+      .. versionchanged:: 3.15
+         Fixed a bug where :class:`bytes` and :class:`str` values ending with
+         ``\x00`` nul bytes or characters were silently stripped when fetching
+         them by index. Trailing nulls are now preserved correctly.
+         See :gh:`106939` and :gh:`145261`.
+
+      In Python 3.14 and earlier, a bug exists where :class:`bytes` and
+      :class:`str` values ending with ``\x00`` nul bytes or characters may be
       *silently stripped* when fetching them by index from the
-      :class:`!ShareableList`. This ``.rstrip(b'\x00')`` behavior is
-      considered a bug and may go away in the future. See :gh:`106939`.
+      :class:`!ShareableList`. This ``.rstrip(b'\x00')`` behavior has been
+      fixed in Python 3.15.
 
-   For applications where rstripping of trailing nulls is a problem,
-   work around it by always unconditionally appending an extra non-0
-   byte to the end of such values when storing and unconditionally
-   removing it when fetching:
+   For applications that need to work with Python 3.14 and earlier where
+   rstripping of trailing nulls is a problem, work around it by always
+   unconditionally appending an extra non-0 byte to the end of such values
+   when storing and unconditionally removing it when fetching:
 
    .. doctest::
 
        >>> from multiprocessing import shared_memory
        >>> nul_bug_demo = shared_memory.ShareableList(['?\x00', b'\x03\x02\x01\x00\x00\x00'])
        >>> nul_bug_demo[0]
-       '?'
+       '?\x00'
        >>> nul_bug_demo[1]
-       b'\x03\x02\x01'
+       b'\x03\x02\x01\x00\x00\x00'
        >>> nul_bug_demo.shm.unlink()
+       >>> # Workaround for Python 3.14 and earlier (not needed in 3.15+):
        >>> padded = shared_memory.ShareableList(['?\x00\x07', b'\x03\x02\x01\x00\x00\x00\x07'])
        >>> padded[0][:-1]
        '?\x00'