Link section in custom scan documentation

Hi,

I propose attached which does:

     the <function>BeginCustomScan</> callback will be invoked to give the
-    custom scan provider a chance to do whatever else is needed.
+    custom scan provider a chance to do whatever else is needed. See
+   <xref linkend="custom-scan-execution"> for more details.

It would be similar to how it's done elsewhere in custom-scan.sgml like:

    Convert a custom path to a finished plan.  The return value will generally
    be a <literal>CustomScan</> object, which the callback must allocate and
    initialize.  See <xref linkend="custom-scan-plan"> for more details.

Thanks,
Amit