Doc: fix a description regarding WAL summarizer on glossary page

Hi,

While searching the definition of auxiliary processes, I noticed that 
the
description of the WAL summarizer is incorrect. Additionally, I think 
it's
better to add a description for the WAL writer similar to other 
Auxiliary
processes. What do you think?

# patch

```
diff --git a/doc/src/sgml/glossary.sgml b/doc/src/sgml/glossary.sgml
index a81c17a869..405fe6dc8b 100644
--- a/doc/src/sgml/glossary.sgml
+++ b/doc/src/sgml/glossary.sgml
@@ -164,6 +164,7 @@
       the <glossterm linkend="glossary-wal-archiver">WAL 
archiver</glossterm>,
       the <glossterm linkend="glossary-wal-receiver">WAL 
receiver</glossterm>
       (but not the <glossterm linkend="glossary-wal-sender">WAL 
senders</glossterm>),
+     the <glossterm linkend="glossary-wal-summarizer">WAL 
summarizer</glossterm>,
       and the <glossterm linkend="glossary-wal-writer">WAL 
writer</glossterm>.
      </para>
     </glossdef>
@@ -2199,7 +2200,7 @@
     <glossterm>WAL summarizer (process)</glossterm>
     <glossdef>
      <para>
-     A special <glossterm linkend="glossary-backend">backend 
process</glossterm>
+     An <glossterm linkend="glossary-auxiliary-proc">auxiliary 
process</glossterm>
       that summarizes WAL data for
       <glossterm linkend="glossary-incremental-backup">incremental 
backups</glossterm>.
      </para>
@@ -2213,7 +2214,8 @@
     <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>
```

# additional information

As mentioned in miscadmin.h, WAL summarizer is one of auxiliary 
processes.
(postgres/src/include/miscadmin.h)
```
	/*
	 * Auxiliary processes. These have PGPROC entries, but they are not
	 * attached to any particular database. There can be only one of each 
of
	 * these running at a time.
	 *
	 * If you modify these, make sure to update NUM_AUXILIARY_PROCS and the
	 * glossary in the docs.
	 */
	B_ARCHIVER,
	B_BG_WRITER,
	B_CHECKPOINTER,
	B_STARTUP,
	B_WAL_RECEIVER,
	B_WAL_SUMMARIZER,
	B_WAL_WRITER,
```
Indeed, it calls InitAuxiliaryProcess() instead of InitProcess().

But, the description of WAL summarizer says that it's one of backend 
processes.
> WAL summarizer (process)
> A special backend process that summarizes WAL data for incremental 
> backups.
https://www.postgresql.org/docs/devel/glossary.html


Regards,
-- 
Masahiro Ikeda
NTT DATA CORPORATION