fix: indicate the maximum amount of memory on macOS (83765)

Fixed following the suggestion #83765 (comment)

Author: Ivo Bellin Salarin

Doc/library/multiprocessing.shared_memory.rst

@@ -69,6 +69,7 @@ copying of data.
       memory block may be larger or equal to the size requested.
       When attaching to an existing shared memory block,
       the *size* parameter is ignored.
+      On macOS, the maximum size of a shared memory block is 140256418463744 bytes.
 
    :param bool track:
       When ``True``, register the shared memory block with a resource