Some publicly exposed functions/properties have no documentation

[A5rocks]

This issue should only really be looked at after #3221 is closed (because this is mainly about what people see in an editor, rather than our documentation). The following attributes are listed in our _type_completeness.json as not having a docstring:

• trio._unix_pipes.FdStream.send_all
• trio._unix_pipes.FdStream.wait_send_all_might_not_block
• trio._unix_pipes.FdStream.receive_some
• trio._unix_pipes.FdStream.close
• trio._unix_pipes.FdStream.aclose
• trio._unix_pipes.FdStream.fileno
• trio._core._io_epoll._EpollStatistics
• trio._channel.MemoryReceiveChannel
• trio._channel.MemoryChannelStatistics
• trio._channel.MemorySendChannel
• trio._core._run.Task
• trio._socket.SocketType
• trio._highlevel_socket.SocketStream.send_all
• trio._highlevel_socket.SocketStream.wait_send_all_might_not_block
• trio._highlevel_socket.SocketStream.send_eof
• trio._highlevel_socket.SocketStream.receive_some
• trio._highlevel_socket.SocketStream.aclose
• trio._subprocess.HasFileno.fileno
• trio._sync.AsyncContextManagerMixin
• trio._sync._HasAcquireRelease.acquire
• trio._sync._HasAcquireRelease.release
• trio._sync._LockImpl
• trio._core._local._NoValue
• trio._core._local.RunVarToken
• trio.socket.gaierror
• trio.socket.herror
• trio._core._mock_clock.MockClock.start_clock
• trio._core._mock_clock.MockClock.current_time
• trio._core._mock_clock.MockClock.deadline_to_sleep_time
• trio.testing._raises_group._ExceptionInfo.exconly
• trio.testing._raises_group._ExceptionInfo.errisinstance
• trio.testing._raises_group._ExceptionInfo.getrepr
• trio.testing._raises_group.RaisesGroup.expected_type

I expect most of these are trivial. There's a couple groups I noticed while copying them over:

• some aren't actually publicly exposed and so we should update check_type_completeness.py to filter them out (e.g. _HasAcquireRelease)
• some should really just inherit documentation from parents (e.g. SocketStream.wait_send_all_might_not_block). I don't know if pyright supports anything like that but it should

I think some of the logic from check_type_completeness.py is kind of strange: it shouldn't filter out docstrings that are available at runtime because... pyright still can't see those?