docs: Update ShareableList documentation to reflect trailing null preservation

Added versionchanged directive for Python 3.15 noting that trailing null
bytes are now preserved in both strings and bytes. Updated doctest example
to show correct behavior and clarified workaround is only needed for 3.14
and earlier.

Author: stefanzetzsche

Doc/library/multiprocessing.shared_memory.rst

@@ -311,19 +311,17 @@ finishes execution.
    existing :class:`!ShareableList`, specify its shared memory block's unique
    name while leaving *sequence* set to ``None``.
 
-   .. note::
+   .. versionchanged:: 3.15
+      Trailing null bytes (``\x00``) in :class:`bytes` and :class:`str` values
+      are now preserved correctly. See :gh:`106939` and :gh:`145261`.
 
-      .. 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`.
+   .. note::
 
-      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 has been
-      fixed in Python 3.15.
+      In Python 3.14 and earlier, a known issue exists for :class:`bytes` and
+      :class:`str` values. If they end with ``\x00`` nul bytes or characters,
+      those may be *silently stripped* when fetching them by index from the
+      :class:`!ShareableList`. This ``.rstrip(b'\x00')`` behavior was fixed
+      in Python 3.15.
 
    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
@@ -333,13 +331,14 @@ finishes execution.
    .. doctest::
 
        >>> from multiprocessing import shared_memory
-       >>> nul_bug_demo = shared_memory.ShareableList(['?\x00', b'\x03\x02\x01\x00\x00\x00'])
-       >>> nul_bug_demo[0]
+       >>> # Python 3.15+: trailing nulls are preserved
+       >>> sl = shared_memory.ShareableList(['?\x00', b'\x03\x02\x01\x00\x00\x00'])
+       >>> sl[0]
        '?\x00'
-       >>> nul_bug_demo[1]
+       >>> sl[1]
        b'\x03\x02\x01\x00\x00\x00'
-       >>> nul_bug_demo.shm.unlink()
-       >>> # Workaround for Python 3.14 and earlier (not needed in 3.15+):
+       >>> sl.shm.unlink()
+       >>> # Workaround for Python 3.14 and earlier:
        >>> padded = shared_memory.ShareableList(['?\x00\x07', b'\x03\x02\x01\x00\x00\x00\x07'])
        >>> padded[0][:-1]
        '?\x00'