_drush_add_commandfiles() needs documentation

[joachim-n]

No description provided.

[greg-1-anderson]

Alas, more of the Drush 8 (and earlier) internal methods should indeed have been better documented. This method is now deprecated (or will be) - maintainer efforts are focused on the new dispatcher, so older methods are unlikely to get the documentation they deserve. We will try to answer specific questions, & could consider contributed PRs that improve docs in these areas.