update help about healing

borg/archiver.py

@@ -973,9 +973,12 @@ class Archiver:
         - Check if archive metadata chunk is present. if not, remove archive from
           manifest.
         - For all files (items) in the archive, for all chunks referenced by these
-          files, check if chunk is present (if not and we are in repair mode, replace
-          it with a same-size chunk of zeros). This requires reading of archive and
-          file metadata, but not data.
+          files, check if chunk is present.
+          If a chunk is not present and we are in repair mode, replace it with a same-size
+          replacement chunk of zeros.
+          If a previously lost chunk reappears (e.g. via a later backup) and we are in
+          repair mode, the all-zero replacement chunk will be replaced by the correct chunk.
+          This requires reading of archive and file metadata, but not data.
         - If we are in repair mode and we checked all the archives: delete orphaned
           chunks from the repo.
         - if you use a remote repo server via ssh:, the archive check is executed on