docs(io): document std/io

[kt3k]

part of #3764

[kt3k]

@iuioiua PTAL

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 5 lines in your changes missing coverage. Please review.

Project coverage is 96.26%. Comparing base (1047864) to head (bc17861).

Files	Patch %	Lines
io/buf_writer.ts	90.90%	3 Missing ⚠️
io/buf_reader.ts	92.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5656   +/-   ##
=======================================
  Coverage   96.25%   96.26%           
=======================================
  Files         470      470           
  Lines       38188    38251   +63     
  Branches     5545     5545           
=======================================
+ Hits        36759    36821   +62     
- Misses       1387     1388    +1     
  Partials       42       42           

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[iuioiua]

Nice! Once we trim the package of its deprecated APIs, we should revisit some symbols. Namely, options which don't have their own interfaces don't display their properties' JSDocs correctly.