Clarify 'internal' feature usage

[marmarta]

Or, to be exact, discourage its use.

fixes QubesOS/qubes-issues#6604

[codecov]

Codecov Report

Merging #184 (7b4ef75) into master (baf5037) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   75.22%   75.22%           
=======================================
  Files          51       51           
  Lines        7645     7645           
=======================================
  Hits         5751     5751           
  Misses       1894     1894           

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