doc: checklist notes

Author: c-dilks

doc/qa.md

@@ -52,7 +52,9 @@ If you are performing a manual QA as part of a cross check, skip to the next sec
 <details>
 <summary>- [ ] cross check run list from trains and from DSTs</summary>
 
-- FIXME
+- use `qtl xtrain` to make sure the list of DST runs is consistent with the list of runs from a train
+  - sometimes there are missing train files
+  - the script also checks for missing DST files (though that should be impossible to happen)
 </details>
 
 <details>