Doc: add detail about EXPLAIN's "Disabled" property

c01743aa4 and later 161320b4b adjusted the EXPLAIN output to show which
plan nodes were chosen despite being disabled by the various enable*
GUCs.  Prior to e22253467, the disabledness of a node was only evident by
a large startup cost penalty.  Since we now explicitly tag disabled nodes
with a boolean property in EXPLAIN, let's add some documentation to
provide some details about why and when disabled nodes can appear in the
plan.

Author: Laurenz Albe, David Rowley
Discussion: https://postgr.es/m/883729e429267214753d5e438c82c73a58c3db5d.camel@cybertec.at

doc/src/sgml/perform.sgml

@@ -578,6 +578,31 @@ WHERE t1.unique1 < 100 AND t1.unique2 = t2.unique2;
     discussed <link linkend="using-explain-analyze">below</link>.
    </para>
 
+   <para>
+    When using the enable/disable flags to disable plan node types, many of
+    the flags only discourage the use of the corresponding plan node and don't
+    outright disallow the planner's ability to use the plan node type.  This
+    is by design so that the planner still maintains the ability to form a
+    plan for a given query.  When the resulting plan contains a disabled node,
+    the <command>EXPLAIN</command> output will indicate this fact.
+
+<screen>
+SET enable_seqscan = off;
+EXPLAIN SELECT * FROM unit;
+
+                       QUERY PLAN
+---------------------------------------------------------
+ Seq Scan on unit  (cost=0.00..21.30 rows=1130 width=44)
+   Disabled: true
+</screen>
+   </para>
+
+   <para>
+    Because the <literal>unit</literal> table has no indexes, there is no
+    other means to read the table data, so the sequential scan is the only
+    option available to the query planner.
+   </para>
+
    <para>
     <indexterm>
      <primary>subplan</primary>