Document histogram bucket behavior when using different units

[sonalgaud12]

This PR updates the documentation for AggregationExplicitBucketHistogram to clarify that default bucket boundaries are optimized for milliseconds and do not automatically adjust when a different unit (e.g., seconds) is set using metric.WithUnit("s")

Issue #5891

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (fd08677) to head (75efa73).
Report is 7 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6438   +/-   ##
=====================================
  Coverage   81.8%   81.8%           
=====================================
  Files        283     283           
  Lines      24927   24928    +1     
=====================================
+ Hits       20401   20402    +1     
  Misses      4121    4121           
  Partials     405     405           

see 2 files with indirect coverage changes

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