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
d3014fff4cd4.
Author: Masahiro Ikeda
Discussion: https://postgr.es/m/
d3a5a4278fd8d9e7a47c6aa4db9e9a39@oss.nttdata.com
Backpatch-through: 15