Document log_transaction_sample_rate as superuser-only

The docs do say which GUCs can be changed only by superusers, but we
forgot to mention this for the new log_transaction_sample_rate. This
GUC was introduced in PostgreSQL 12, so backpatch accordingly.

Author: Adrien Nayrat
Backpatch-through: 12

doc/src/sgml/config.sgml

@@ -5988,6 +5988,7 @@ local0.*    /var/log/postgresql
          logs all statements for all transactions.
          <varname>log_transaction_sample_rate</varname> is helpful to track a
          sample of transaction.
+         Only superusers can change this setting.
         </para>
        <note>
         <para>