fix: don't export cimpl_free to C and update docs

[gpeacock]

removes extern C from cimpl_free declaration.
update docs to not claim that we do leak reporting on the static PointerRegistry.
(need to come up with a new way to handle that)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.15%. Comparing base (887e635) to head (d632721).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   65.15%   65.15%           
=======================================
  Files           2        2           
  Lines         815      815           
=======================================
  Hits          531      531           
  Misses        284      284           

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