Document relminmxid and datminmxid

I introduced these new fields in 0ac5ad51 but neglected to add them
to the system catalogs section of the docs.

Per Thom Brown in message
CAA-aLv7UiO=Whiq3MVbsEqSyQRthuX8Tb_RLyBuQt0KQBp=6EQ@mail.gmail.com

doc/src/sgml/catalogs.sgml

@@ -1884,6 +1884,19 @@
       </entry>
      </row>
 
+     <row>
+      <entry><structfield>relminmxid</structfield></entry>
+      <entry><type>xid</type></entry>
+      <entry></entry>
+      <entry>
+       All multitransaction IDs before this one have been replaced by a
+       transaction ID in this table.  This is used to track
+       whether the table needs to be vacuumed in order to prevent multitransaction ID
+       ID wraparound or to allow <literal>pg_clog</> to be shrunk.  Zero
+       (<symbol>InvalidTransactionId</symbol>) if the relation is not a table.
+      </entry>
+     </row>
+
      <row>
       <entry><structfield>relacl</structfield></entry>
       <entry><type>aclitem[]</type></entry>
@@ -2621,6 +2634,20 @@
       </entry>
      </row>
 
+     <row>
+      <entry><structfield>datminmxid</structfield></entry>
+      <entry><type>xid</type></entry>
+      <entry></entry>
+      <entry>
+       All multitransaction IDs before this one have been replaced with a
+       transaction ID in this database.  This is used to
+       track whether the database needs to be vacuumed in order to prevent
+       transaction ID wraparound or to allow <literal>pg_clog</> to be shrunk.
+       It is the minimum of the per-table
+       <structname>pg_class</>.<structfield>relminmxid</> values.
+      </entry>
+     </row>
+
      <row>
       <entry><structfield>dattablespace</structfield></entry>
       <entry><type>oid</type></entry>