[chore] Clarify configopaque expectations

[mx-psi]

Description:

Documents configopaque expectations.

The intention is that it is clear that we can add interface implementations after 1.0 if not doing so would leak opaque values.

Link to tracking Issue: Fixes #9274

[mx-psi]

cc @TylerHelmuth

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (440bb82) 90.31% compared to head (27010a1) 90.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9295      +/-   ##
==========================================
+ Coverage   90.31%   90.33%   +0.01%     
==========================================
  Files         341      341              
  Lines       18308    18308              
==========================================
+ Hits        16535    16538       +3     
+ Misses       1437     1435       -2     
+ Partials      336      335       -1     

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