Doc fix for RecordException

[cijothomas]

Fixes #1569

Given we were using ToString() from the 1st stable release, documenting that ex.ToString() is the chosen representation of stacktrace. #1569 has question about using ToString() vs StackTrace, but given this is already part of stable release, we need to keep this behavior.

It is okay to provide an option to customize this, if there is demand, or change the chosen representation in v2.0. This PR is just documenting the existing behavior.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #5143 (1074390) into main (58e8f24) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5143   +/-   ##
=======================================
  Coverage   83.29%   83.29%           
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  Files         297      297           
  Lines       12380    12380           
=======================================
  Hits        10312    10312           
  Misses       2068     2068           

Flag	Coverage Δ
unittests	83.29% <ø> (ø)

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Files	Coverage Δ
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	95.45% <ø> (ø)

... and 8 files with indirect coverage changes