add auth.email function definition in auth deep dive

web/docs/learn/auth-deep-dive/policies.md

@@ -136,6 +136,10 @@ There are some more notes here on how to structure your schema to best integrate
 Once you get the hang of polices you can start to get a little bit fancy. Let's say I work at Blizzard and I only want Blizzard staff members to be able to update people's high scores, I can write something like:
 
 ```sql
+create or replace function auth.email() returns text as $$
+  select nullif(current_setting('request.jwt.claim.email', true), '')::text;
+$$ language sql;
+
 create policy "Only Blizzard staff can update leaderboard"
   on my_scores
   for update using (