[otlp] Nullable annotations for the OtlpExporterOptions class and some xml doc improvement

[CodeBlanch]

[Originally part of #5400]

Changes

• Add nullable annotations for the OtlpExporterOptions class
• Tweaks some of the XML docs for clarity

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.89%. Comparing base (6250307) to head (82951f8).
Report is 125 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5434      +/-   ##
==========================================
+ Coverage   83.38%   84.89%   +1.50%     
==========================================
  Files         297      279      -18     
  Lines       12531    11556     -975     
==========================================
- Hits        10449     9810     -639     
+ Misses       2082     1746     -336     

Flag	Coverage Δ
unittests	?
unittests-Solution-Experimental	84.90% <100.00%> (?)
unittests-Solution-Stable	84.83% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...orter.OpenTelemetryProtocol/OtlpExporterOptions.cs	97.05% <100.00%> (+0.69%)	⬆️
...lemetryProtocol/OtlpLogExporterHelperExtensions.cs	71.06% <100.00%> (ø)

... and 80 files with indirect coverage changes