docs: explain the frozen input default content scope

README.md

@@ -362,6 +362,7 @@ Public reference hashes do not replace those inputs.
 
 Use `webstone validate --dataset <exported-dataset.json>
 --content-scope main` or `--content-scope full` before a run. Each run selects one
-scope; omitted selection defaults to `full` for this annotated suite. Older suites
-keep their original rules. The existing manifest tooling pin still describes the
+scope. The approved input bound by this cohort declares `main` as its default.
+An explicit option overrides a saved scope; annotated tasks without a saved scope
+fall back to `full`. Older suites keep their original rules. The existing manifest tooling pin still describes the
 released edition; the cohort's `implementation` field identifies its scoring code.