Add a comment on the migration stating that we keep an history of the policy data

crates/storage-pg/migrations/20250225091000_dynamic_policy_data.sql

@@ -4,6 +4,10 @@
 -- Please see LICENSE in the repository root for full details.
 
 -- Add a table which stores the latest policy data
+--
+-- Every time the policy data is updated, it creates a new row, so that we keep
+-- an history of the policy data, trace back which version of the data was used
+-- on each evaluation.
 CREATE TABLE IF NOT EXISTS policy_data (
     policy_data_id UUID PRIMARY KEY,
     created_at TIMESTAMP WITH TIME ZONE NOT NULL,