Clarify character helper semantics

[Jim8y]

Summary

• document ASCII-oriented char and string helper semantics
• clarify that char.GetNumericValue does not implement full .NET Unicode numeric behavior
• link C# 1 syntax notes to the semantic differences page

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 (637f578).

Additional details and impacted files

@@            Coverage Diff             @@
##           master-n3    #1593   +/-   ##
==========================================
  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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