Clarify using declaration IO support

[Jim8y]

Summary

• clarify that using declaration syntax support does not imply System.IO API support
• replace the MemoryStream example with a contract-compatible IDisposable example

Validation

• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.24%. Comparing base (3665f83) to head (459483b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master-n3    #1589   +/-   ##
==========================================
  Coverage      80.24%   80.24%           
==========================================
  Files            295      295           
  Lines          25456    25456           
  Branches        3453     3453           
==========================================
  Hits           20428    20428           
  Misses          4132     4132           
  Partials         896      896           

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