Document AUTHORIZED_FETCH mode and WHITELIST_MODE (#12856)

* Document AUTHORIZED_FETCH mode and WHITELIST_MODE

* Replace extended description with a link to the online docs

.env.production.sample

@@ -250,3 +250,13 @@ STREAMING_CLUSTER_NUM=1
 # http_proxy=http://gateway.local:8118
 # Access control for hidden service.
 # ALLOW_ACCESS_TO_HIDDEN_SERVICE=true
+
+# Authorized fetch mode (optional)
+# Require remote servers to authentify when fetching toots, see
+# https://docs.joinmastodon.org/admin/config/#authorized_fetch
+# AUTHORIZED_FETCH=true
+
+# Whitelist mode (optional)
+# Only allow federation with whitelisted domains, see
+# https://docs.joinmastodon.org/admin/config/#whitelist_mode
+# WHITELIST_MODE=true