Update public-api.asciidoc

[andremm]

I believe we should use ElasticApm.startTransaction() and ElasticApm.startSpan() for creating custom transactions and spans. Please correct me if I'm mistaken.

[codecov-io]

Codecov Report

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@@         Coverage Diff          @@
##             0.6   #160   +/-   ##
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  Coverage       ?    77%           
  Complexity     ?    957           
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  Files          ?     99           
  Lines          ?   3301           
  Branches       ?    293           
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  Hits           ?   2542           
  Misses         ?    602           
  Partials       ?    157

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