Add docstrings for memory channel classes

[Madhur-7568]

Summary

Add class docstrings for:

• MemorySendChannel
• MemoryReceiveChannel
• MemoryChannelStatistics

This improves the generated API documentation for these classes.

[A5rocks]

Please appease CI! I'm not too sure about these new docstrings. I'm not sure my review comments are correct, but I think they would be more useful to readers at least.

[A5rocks]

Thanks for the changes. Just a couple nitpicks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (865b7fc) to head (cbfea11).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3465   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19417        19423    +6     
  Branches         1317         1317           
===============================================
+ Hits            19417        19423    +6     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Madhur-7568]

I've made all the changes. Please review when you have time. Thanks!

[Madhur-7568]

Hi! Thanks again for reviewing and approving the PR. Just wanted to check if there's anything else needed from my side before it can be merged. Thanks!

[A5rocks]

Nothing else is necessary from you, I'm just waiting for someone else to approve/merge.

[CoolCat467]

"for python objects" seemed a little weird but I can't think of a better description myself, looks good.