make use of _simple_select_one_onecol, improved comments

synapse/storage/schema/delta/51/monthly_active_users.sql

@@ -16,6 +16,10 @@
 -- a table of monthly active users, for use where blocking based on mau limits
 CREATE TABLE monthly_active_users (
     user_id TEXT NOT NULL,
+    -- Last time we saw the user. Not guaranteed to be accurate due to rate limiting
+    -- on updates, Granularity of updates governed by
+    -- syanpse.storage.monthly_active_users.LAST_SEEN_GRANULARITY
+    -- Measured in ms since epoch.
     timestamp BIGINT NOT NULL
 );