docs: Added comments for 3 private functions (fixes #371)

[juliafeldhousen]

Added documentation for three undocumented private functions: lock_param() in param.h, cap_size_t_to_int() in inthelper.h, and copy_cstring() in strhelper.h. This is a part of ongoing work on issue #371. Let me know if any changes are needed!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.64%. Comparing base (d843cb0) to head (bfc2161).
⚠️ Report is 1 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest     #546      +/-   ##
==========================================
- Coverage   90.82%   90.64%   -0.18%     
==========================================
  Files          47       47              
  Lines        4566     4566              
  Branches      605      605              
==========================================
- Hits         4147     4139       -8     
- Misses        276      283       +7     
- Partials      143      144       +1     

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[goatshriek]

Thanks very much for putting this together!