Update docs/public-api.asciidoc

[gregkalapos]

Following up on #231 - there was some surprise that Elastic.Apm.Agent.Tracer.CurrentTransaction can be null. We mention this where CurrentTransaction is documented, nevertheless I think a similar note in the CaptureSpan part does not hurt, since we heavily use CurrentTransaction in the code samples.

Plus I promised to add such a note here.

[bmorelli25]

👍

[SergeyKleyman]

LGTM

[codecov-io]

Codecov Report

Merging #332 into master will increase coverage by 2.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   75.55%   78.09%   +2.53%     
==========================================
  Files          84       72      -12     
  Lines        2974     2465     -509     
  Branches      580      439     -141     
==========================================
- Hits         2247     1925     -322     
+ Misses        557      361     -196     
- Partials      170      179       +9

Impacted Files	Coverage Δ
src/Elastic.Apm/Model/Error.cs	66.66% <0%> (-18.52%)	⬇️
src/Elastic.Apm/Metrics/MetricsCollector.cs	77.27% <0%> (-9.65%)	⬇️
src/Elastic.Apm/Report/PayloadSenderV2.cs	67.64% <0%> (-6.81%)	⬇️
...c/Elastic.Apm.AspNetCore/ApmMiddlewareExtension.cs	45.16% <0%> (-5.91%)	⬇️
src/Elastic.Apm/Model/Span.cs	95.95% <0%> (-4.05%)	⬇️
src/Elastic.Apm/Helpers/SystemInfoHelper.cs	77.04% <0%> (-1.64%)	⬇️
src/Elastic.Apm/Model/Transaction.cs	97.32% <0%> (-0.53%)	⬇️
....Apm/Report/Serialization/PayloadItemSerializer.cs	100% <0%> (ø)	⬆️
src/Elastic.Apm/Helpers/ObjectExtensions.cs	100% <0%> (ø)	⬆️
...pm.Elasticsearch/SerializerDiagnosticsListener .cs
... and 13 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f58271c...79badf3. Read the comment docs.