Add missing documentation for argument of amcostestimate()

5262f7a4 have introduced parallel index scan.  In order to estimate the
number of parallel workers, it adds extra argument to amcostestimate() index
access method API function.  However, this extra argument was missed in the
documentation.  This commit fixes that.

Discussion: https://postgr.es/m/4128fdb4-8b63-2e05-38f6-3125f8c27263%40lab.ntt.co.jp
Author: Tatsuro Yamada, Alexander Korotkov
Backpatch-through: 10

doc/src/sgml/indexam.sgml

@@ -385,7 +385,8 @@ amcostestimate (PlannerInfo *root,
                 Cost *indexStartupCost,
                 Cost *indexTotalCost,
                 Selectivity *indexSelectivity,
-                double *indexCorrelation);
+                double *indexCorrelation,
+                double *indexPages);
 </programlisting>
    Estimate the costs of an index scan.  This function is described fully
    in <xref linkend="index-cost-estimation"/>, below.
@@ -1155,7 +1156,8 @@ amcostestimate (PlannerInfo *root,
                 Cost *indexStartupCost,
                 Cost *indexTotalCost,
                 Selectivity *indexSelectivity,
-                double *indexCorrelation);
+                double *indexCorrelation,
+                double *indexPages);
 </programlisting>
 
    The first three parameters are inputs:
@@ -1197,7 +1199,7 @@ amcostestimate (PlannerInfo *root,
   </para>
 
   <para>
-   The last four parameters are pass-by-reference outputs:
+   The last five parameters are pass-by-reference outputs:
 
    <variablelist>
     <varlistentry>
@@ -1236,6 +1238,15 @@ amcostestimate (PlannerInfo *root,
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><parameter>*indexPages</parameter></term>
+     <listitem>
+      <para>
+       Set to number of index leaf pages
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
@@ -1283,6 +1294,11 @@ amcostestimate (PlannerInfo *root,
    table.
   </para>
 
+  <para>
+   The <parameter>indexPages</parameter> should be set to the number of leaf pages.
+   This is used to estimate the number of workers for parallel index scan.
+  </para>
+
   <para>
    When <parameter>loop_count</parameter> is greater than one, the returned numbers
    should be averages expected for any one scan of the index.