Say what the content phase's check does to a stale record

The README content phase item and the contentCandidates comment said a
file failing the check is not a duplicate. It keeps its record and does
not count as a match; with a content hash it is still reported until a
scan covering its tree updates or removes it.

Model: opus-5-5

scan.go

@@ -607,11 +607,14 @@ func (s *scanState) contentPhase(ctx context.Context, workers int) error {
 // their records by path. Every record contentCandidatesSQL returns has
 // its file checked with lstat, whether or not it already has a content
 // hash: a file that is gone, is no longer a regular file, or has
-// changed by the walk's rule keeps its record as it is and is not a
-// duplicate, and any other lstat error is warned about and counted as
-// skipped. The files of a group that pass and have no content hash are
-// read only if at least minGroupSize of the group's files pass, so a
-// group whose other members are all stale costs no reads. Only the
+// changed by the walk's rule keeps its record as it is and does not
+// count as a match for the others, and any other lstat error is warned
+// about and counted as skipped, with the same result. If such a record
+// has no content hash, it stays out of duplicate groups; if it has one,
+// it is still reported until a scan covering its own tree updates or
+// removes it. The files of a group that pass and have no content hash
+// are read only if at least minGroupSize of the group's files pass, so
+// a group whose other members are all stale costs no reads. Only the
 // records to be read are kept.
 func (s *scanState) contentCandidates(
 	ctx context.Context,