Add notes to basics section on underflow, overflow, and non-finite values handling

[mborland]

Closes: #1088

[cppalliance-bot]

An automated preview of the documentation is available at https://1090.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2025-10-07 06:51:51 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.4%. Comparing base (460fde2) to head (4a25581).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1090     +/-   ##
=========================================
- Coverage     99.4%   99.4%   -0.0%     
=========================================
  Files          244     244             
  Lines        16743   16743             
  Branches      1812    1812             
=========================================
- Hits         16638   16637      -1     
- Misses         105     106      +1     

see 1 file with indirect coverage changes

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