docs: add missing docstrings for public API

[nightcityblade]

Fixes #3221

Adds docstrings to the following public API items that were showing up without documentation:

• MemorySendChannel — class docstring referencing open_memory_channel
• MemoryReceiveChannel — class docstring referencing open_memory_channel
• MemoryChannelStatistics — class docstring with attribute descriptions
• SocketStream.send_all, wait_send_all_might_not_block, send_eof, receive_some, aclose — short docstrings referencing the parent ABC methods
• HasFileno.fileno — method docstring
• ParkingLot.broken_by — attribute docstring

The SocketStream method docstrings use See :meth:... cross-references rather than duplicating the ABC documentation, keeping them in sync with the parent class.

Tests run: pytest src/trio/_tests/test_channel.py src/trio/_tests/test_highlevel_socket.py src/trio/_core/_tests/test_parking_lot.py — all pass.

[jakkdl]

See discussion in #3388

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (3dd35d7) to head (3da0288).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3404   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19424        19425    +1     
  Branches         1318         1318           
===============================================
+ Hits            19424        19425    +1     

Files with missing lines	Coverage Δ
src/trio/_channel.py	100.00000% <ø> (ø)
src/trio/_core/_parking_lot.py	100.00000% <100.00000%> (ø)
src/trio/_highlevel_socket.py	100.00000% <ø> (ø)
src/trio/_subprocess.py	100.00000% <ø> (ø)

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