Fix doc oversight for custom WAL resource managers.

Reported-by: Bharath Rupireddy
Backpatch-through: 15
Discussion: https://postgr.es/m/CALj2ACU+at7RqnWEzS59QsFg3ZOF4C4GSp7pt+PWiLEp0zrEKg@mail.gmail.com

doc/src/sgml/tableam.sgml

@@ -87,11 +87,8 @@
   linkend="wal"><acronym>WAL</acronym></link>, or a custom implementation.
   If <acronym>WAL</acronym> is chosen, either <link
   linkend="generic-wal">Generic WAL Records</link> can be used,
-  or a new type of <acronym>WAL</acronym> records can be implemented.
-  Generic WAL Records are easy, but imply higher WAL volume.
-  Implementation of a new type of WAL record
-  currently requires modifications to core code (specifically,
-  <filename>src/include/access/rmgrlist.h</filename>).
+  or a <link linkend="custom-rmgr">Custom WAL Resource Manager</link> can be
+  implemented.
  </para>
 
  <para>