Document I/O buffer library

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Rendered

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.87%. Comparing base (496432e) to head (350e947).
Report is 8 commits behind head on main.

❗ Current head 350e947 differs from pull request most recent head 271deed

Please upload reports for the commit 271deed to get more accurate results.

Files	Patch %	Lines
amaranth/lib/io.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1364      +/-   ##
==========================================
- Coverage   90.99%   90.87%   -0.13%     
==========================================
  Files          45       45              
  Lines       11218    11208      -10     
  Branches     2429     2361      -68     
==========================================
- Hits        10208    10185      -23     
- Misses        831      850      +19     
+ Partials      179      173       -6     

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