Doc: improve documentation about run-time pruning's effects on EXPLAIN.

Tatsuo Ishii complained that this para wasn't very intelligible.
Try to make it better.

Discussion: https://postgr.es/m/20191207.200500.989741087350666720.t-ishii@sraoss.co.jp

doc/src/sgml/perform.sgml

@@ -896,15 +896,16 @@ EXPLAIN ANALYZE SELECT * FROM tenk1 WHERE unique1 < 100 AND unique2 > 9000
    </para>
 
    <para>
-    Generally, the <command>EXPLAIN</command> output will display details for
-    every plan node which was generated by the query planner.  However, there
-    are cases where the executor is able to determine that certain nodes are
-    not required; currently, the only node types to support this are the
-    <literal>Append</literal> and <literal>MergeAppend</literal> nodes.  These
-    node types have the ability to discard subnodes which they are able to
-    determine won't contain any records required by the query.  It is possible
-    to determine that nodes have been removed in this way by the presence of a
-    "Subplans Removed" property in the <command>EXPLAIN</command> output.
+    Normally, <command>EXPLAIN</command> will display every plan node
+    created by the planner.  However, there are cases where the executor
+    can determine that certain nodes need not be executed because they
+    cannot produce any rows, based on parameter values that were not
+    available at planning time.  (Currently this can only happen for child
+    nodes of an Append or MergeAppend node that is scanning a partitioned
+    table.)  When this happens, those plan nodes are omitted from
+    the <command>EXPLAIN</command> output and a <literal>Subplans
+    Removed: <replaceable>N</replaceable></literal> annotation appears
+    instead.
    </para>
   </sect2>