docs: remove unnecessary backrefs and interface mentions

Address review feedback:
- Remove backref in MemoryChannelStatistics docstring
- Remove "implements interface" lines from Send/ReceiveChannel

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: wavebyrd

src/trio/_channel.py

@@ -120,9 +120,6 @@ def __init__(self, max_buffer_size: int | float) -> None:  # noqa: PYI041
 class MemoryChannelStatistics:
     """Statistics about a memory channel.
 
-    This object is returned by the ``statistics()`` method on
-    :class:`MemorySendChannel` and :class:`MemoryReceiveChannel`.
-
     Attributes:
         current_buffer_used: The number of items currently stored in the
             channel buffer.
@@ -177,8 +174,6 @@ class MemorySendChannel(SendChannel[SendType], metaclass=NoPublicConstructor):
     Memory channels are created using :func:`open_memory_channel`, which
     returns a pair of (:class:`MemorySendChannel`, :class:`MemoryReceiveChannel`).
     See :func:`open_memory_channel` for full documentation.
-
-    This implements the :class:`~trio.abc.SendChannel` interface.
     """
 
     _state: MemoryChannelState[SendType]
@@ -334,8 +329,6 @@ class MemoryReceiveChannel(ReceiveChannel[ReceiveType], metaclass=NoPublicConstr
     Memory channels are created using :func:`open_memory_channel`, which
     returns a pair of (:class:`MemorySendChannel`, :class:`MemoryReceiveChannel`).
     See :func:`open_memory_channel` for full documentation.
-
-    This implements the :class:`~trio.abc.ReceiveChannel` interface.
     """
 
     _state: MemoryChannelState[ReceiveType]