document vendored feature of openssl

[gregwebs]

This helps with cross compilation

✓ Checklist:

• This PR has been added to CHANGELOG.md (at the top of the list);
• Docs have been added / updated (for bug fixes / features).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f35ccb) 87.10% compared to head (287fb34) 87.10%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #207   +/-   ##
=======================================
  Coverage   87.10%   87.10%           
=======================================
  Files          60       60           
  Lines        4173     4173           
=======================================
  Hits         3635     3635           
  Misses        538      538           

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