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