docs: gpbackup/gprestore - partitioned table with external table partition not supported (#3879)

* docs: gpbackup/gprestore limitation for partitioned table with external table partition.

* docs: edit to fix a typo.

* docs: updated gpbackup/gprestore limitation. partitioned table w/ external table leaf part. not supported.

gpdb-doc/dita/admin_guide/managing/backup-gpbackup.xml

@@ -45,12 +45,8 @@
             inherited from the new parent table. In this case, <codeph>gpbackup</codeph> backs up
             conflicting <codeph>CREATE INDEX</codeph> statements, which causes an error when you
             restore the backup set.</li>
-          <li>You cannot back up and restore multi-level partitioned tables that have been altered
-            by exchanging a leaf child partition with a readable external table. You can back up and
-            restore single-level partitioned tables when a leaf child partition is a readable
-            external table if the partitions are named partitions (<codeph>partitionname</codeph> in
-            the <codeph>pg_partitions</codeph> system view is not <codeph>NULL</codeph> for the
-            partition).</li>
+          <li>You cannot back up and restore partitioned tables that have been altered by exchanging
+            a leaf child partition with a readable external table. </li>
           <li>You can execute multiple instances of <codeph>gpbackup</codeph>, but each execution
             requires a distinct timestamp.</li>
           <li>Database object filtering is currently limited to schemas and tables.</li>