docs(cuda.core): add AccessedBySet to api_private.rst (R5)

Per Andy's review on PR #1775 (_managed_buffer.py:52), document
`AccessedBySet` in the private API reference. It is returned by
`ManagedBuffer.accessed_by` but not directly instantiable by users —
matches the existing `_memory._ipc.*` entries in the same section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rparolin

cuda_core/docs/source/api_private.rst

@@ -32,6 +32,7 @@ CUDA runtime
    _device.DeviceProperties
    _memory._ipc.IPCAllocationHandle
    _memory._ipc.IPCBufferDescriptor
+   _memory._managed_buffer.AccessedBySet
 
 
 CUDA graphs