Add mention of encryption secrets to production sample (#32512)

.env.production.sample

@@ -45,6 +45,16 @@ ES_PASS=password
 SECRET_KEY_BASE=
 OTP_SECRET=
 
+# Encryption secrets
+# ------------------
+# Must be available (and set to same values) for all server processes
+# These are private/secret values, do not share outside hosting environment
+# Use `bin/rails db:encryption:init` to generate fresh secrets
+# ------------------
+# ACTIVE_RECORD_ENCRYPTION_DETERMINISTIC_KEY=
+# ACTIVE_RECORD_ENCRYPTION_KEY_DERIVATION_SALT=
+# ACTIVE_RECORD_ENCRYPTION_PRIMARY_KEY=
+
 # Web Push
 # --------
 # Generate with `bundle exec rails mastodon:webpush:generate_vapid_key`