db: add better comment about migration

db/migrations/0011_case_insensitive_emails.sql

 -- +migrate Up
 
+-- This migration is a fix for a bug that allowed duplicate emails if they used different cases (see #338).
+-- When migrating, dex will not take the liberty of deleting rows for duplicate cases. Instead it will
+-- raise an exception and call for an admin to remove duplicates manually.
+
 CREATE OR REPLACE FUNCTION raise_exp() RETURNS VOID AS $$
 BEGIN
      RAISE EXCEPTION 'Found duplicate emails when using case insensitive comparision, cannot perform migration.';

db/user.go

@@ -425,7 +425,7 @@ func (r *userRepo) insertRemoteIdentity(tx repo.Transaction, userID string, ri u
 
 type userModel struct {
 	ID            string `db:"id"`
-	Email         string `db:"email"` // NOTE(ericchiang): When making comparisions emails are case insensitive.
+	Email         string `db:"email"`
 	EmailVerified bool   `db:"email_verified"`
 	DisplayName   string `db:"display_name"`
 	Disabled      bool   `db:"disabled"`