[8.14](backport #4167) [apm] Add examples using trace_continuation_strategy to sampling docs

[mergify]

Fixes #3245

Adds information to Transaction sampling about using trace_continuation_strategy for more control over how sampling works when traces include services that aren't Elastic-monitored, but do have a traceparent header.

Questions:

• I don't know if these examples/diagrams are correct, but I made my best guess after reading through GitHub issues and APM agent docs. @bmorelli25 can you help me find the right person to review?
• Does it also make sense to add related info to Distributed tracing?
• Should I add links to the APM agent docs in Configure head-based sampling, too?

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  This is an automatic backport of pull request [apm] Add examples using trace_continuation_strategy to sampling docs #4167 done by Mergify.

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