docs: explain LIMIT 10 clause in keyshare user email job

server/keyshare/tasks/tasks.go

@@ -116,6 +116,9 @@ func (t *taskHandler) expireAccounts() {
 	// Iterate over users we havent seen in ExpiryDelay days, and which have a registered email.
 	// We ignore (and thus keep alive) accounts without email addresses, as we can't inform their owners.
 	// (Note that for such accounts we store no email addresses, i.e. no personal data whatsoever.)
+	// We do this for only 10 users at a time to prevent us from sending out lots of emails
+	// simultaneously, which could lead to our email server being flagged as sending spam.
+	// The users excluded by this limit will get their email next time this task is executed.
 	err := t.db.QueryIterate(`
 		SELECT id, username, language
 		FROM irma.users