Restore FrozenDocumentLoader overview

Author: anatoly-scherbakov

docs/reference/document-loaders/frozen.md

@@ -7,6 +7,10 @@
       heading_level: 3
       members: false
 
+`FrozenDocumentLoader` serves only URLs in an allowlist and refuses all other
+document loads. It is intended for air-gapped runs, reproducible builds, and
+deployments that must avoid remote context fetching.
+
 With no arguments, the loader serves the curated `BUNDLED_CONTEXTS` mapping:
 
 {{ example('document_loaders/frozen_default.py', output_syntax='json') }}