build: comment why partial build cannot be an error

Issue aurutils#1218

Author: AladW

lib/pacman/aur-build

@@ -388,6 +388,9 @@ while IFS= read "${read_args[@]}" -ru "$fd" path; do
             # package group is unavailable (including -debug packages)
             printf >&2 '%s: warning: package group partially built\n' "$argv0"
             printf '%q\n' >&2 "${exists[@]}"
+            # This is necessarily a warning, not an error, since makepkg
+            # unconditionally prints `-debug` packages even when they will
+            # later not be created (#1218)
         fi
 
         if (( ${#exists[@]} )) && [[ -v results_file ]]; then