pg_upgrade: document possible pg_hba.conf options

Previously, pg_upgrade docs recommended using .pgpass if using MD5
authentication to avoid being prompted for a password.  Turns out pg_ctl
never prompts for a password, so MD5 requires .pgpass --- document that.
Also recommend 'peer' for authentication too.
Backpatch back to 9.1.

doc/src/sgml/pgupgrade.sgml

@@ -253,10 +253,10 @@ gmake prefix=/usr/local/pgsql.new install
 
     <para>
      <command>pg_upgrade</> will connect to the old and new servers several times,
-     so you might want to set authentication to <literal>trust</> in
-     <filename>pg_hba.conf</>, or if using <literal>md5</> authentication,
-     use a <filename>~/.pgpass</> file (see <xref linkend="libpq-pgpass">)
-     to avoid being prompted repeatedly for a password.
+     so you might want to set authentication to <literal>trust</>
+     or <literal>peer</> in <filename>pg_hba.conf</>, or if using
+     <literal>md5</> authentication, use a <filename>~/.pgpass</> file
+     (see <xref linkend="libpq-pgpass">).
     </para>
    </step>