make an explicit header for the username policy

tsa/doc/accounts.creole

 [[!meta title="git, shell, ldap, etc. accounts"]]
 
-[[!toc levels=3]]
+[[!toc levels=4]]
 
 = torproject.org Accounts =
 
@@ -91,6 +91,8 @@ in the {{{Tor Sysadmin Team}}} component:
   file for them.  Please enclose the OpenPGP clearsigned blob using
   {{{{{{}}} and {{{}}}}}}.
 
+==== username policy ====
+
 Usernames are allocated on a first-come, first-served basis. Usernames
 should be checked for conflict with commonly used adminstrative
 aliases (`root`, `abuse`, ...) or abusive names (`killall*`, ...). In