fix: Clarify the meaning of aggregate performance metrics in quera

[peterkomar-aws]

Issue #, if available:

Description of changes:
Updates to the docstrings of the aggregate performance metrics of the QuEra device. Now they explicitly explain that these peformance measures correspond to the 4-microsecond mark in the quantum evolution.

Testing done:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (76fa8c8) to head (870b9ce).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files          102       102           
  Lines         1481      1481           
  Branches        17        17           
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  Hits          1481      1481           

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