doc: Fix description WAL writer in glossary

The WAL writer is an auxiliary process, but its description in the
glossary did not match that.

This is inexact since d3014fff4c.

Author: Masahiro Ikeda
Discussion: https://postgr.es/m/d3a5a4278fd8d9e7a47c6aa4db9e9a39@oss.nttdata.com
Backpatch-through: 15

doc/src/sgml/glossary.sgml

@@ -2028,7 +2028,8 @@
    <glossterm>WAL writer (process)</glossterm>
    <glossdef>
     <para>
-     A process that writes <glossterm linkend="glossary-wal-record">WAL records</glossterm>
+     An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm>
+     that writes <glossterm linkend="glossary-wal-record">WAL records</glossterm>
      from <glossterm linkend="glossary-shared-memory">shared memory</glossterm> to
      <glossterm linkend="glossary-wal-file">WAL files</glossterm>.
     </para>