This naming scheme supports auto-detection and on-demand loading of completions.
See
ba109693ee/README.md (faq),
section "Where should I put it to be sure that interactive bash shells will find it and source it".
Previously, distro package maintainers had to rename these files manually.
Completions are dynamically generated from the respective binary help
pages.
Completions should be sourced into the shell or added to
$XDG_CONFIG/fish/completions.