REPO_POLICIES: say which lines an ignore file lacking the comment keeps

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -951,7 +951,9 @@ style conventions are in separate documents:
     files is at the top of `script/install`. The script replaces each file and
     keeps only the repo's own entries:
     - in `.gitignore`, `.editorconfig` and `.dockerignore`: the lines below the
-      comment that ends the canonical file
+      comment that ends the canonical file, and, in a `.gitignore` or
+      `.dockerignore` lacking that comment, every line the canonical file has
+      never held
     - in `.golangci.yml`: the entries the repo added to the `deny` list of the
       `test-support` depguard rule