Document that effective cache size does not assume data remains in the

cache between queries.

Author: bmomjian

doc/src/sgml/config.sgml

@@ -2424,7 +2424,9 @@ SET ENABLE_SEQSCAN TO OFF;
         space.  This parameter has no effect on the size of shared
         memory allocated by <productname>PostgreSQL</productname>, nor
         does it reserve kernel disk cache; it is used only for estimation
-        purposes.  The default is 128 megabytes (<literal>128MB</>).
+        purposes.  The system also does not assume data remains in
+        the disk cache between queries.  The default is 128 megabytes
+        (<literal>128MB</>).
        </para>
       </listitem>
      </varlistentry>