[Docs] Clarify description for propagation_extract_first

[brett0000FF]

What does this PR do?

Update description for tracing.distributed_tracing.propagation_extract_first.

Motivation:

Feedback that "exiting" is confusing.

Additional Notes:

How to test the change?

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.91%. Comparing base (849c60c) to head (bdafc58).
Report is 9 commits behind head on master.

Additional details and impacted files

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  Coverage   97.91%   97.91%           
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  Files        1241     1241           
  Lines       74632    74692   +60     
  Branches     3605     3605           
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+ Hits        73074    73134   +60     
  Misses       1558     1558           

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[p-datadog]

The change proposed in this PR makes sense to me, I would additionally mention what would happen if the "extract first" setting is not set (which format would be used if there are multiple ones found).

[brett0000FF]

The change proposed in this PR makes sense to me, I would additionally mention what would happen if the "extract first" setting is not set (which format would be used if there are multiple ones found).

@p-datadog - Good feedback! I agree it'd be helpful to include information for when this option is set to false. In that case, would it use the last valid propagation format it finds? I'm not sure, so any clarification would be helpful. Thanks!

[p-datadog]

@brett0000FF I actually don't know which is in part why I mentioned this (I do not work on tracer).