doc: Add "(process)" to the term "WAL receiver" in glossary.

This commit changes the term "WAL receiver" to "WAL receiver (process)"
in the glossary, so that users can easily understand WAL receiver is
obviously a process.

Author: Fujii Masao
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/51b76596-ab4f-c9f0-1005-60ce1bb14b6b@oss.nttdata.com

Author: MasaoFujii

doc/src/sgml/glossary.sgml

@@ -1972,7 +1972,7 @@
   </glossentry>
 
   <glossentry id="glossary-wal-receiver">
-   <glossterm>WAL receiver</glossterm>
+   <glossterm>WAL receiver (process)</glossterm>
    <glossdef>
     <para>
      An <glossterm linkend="glossary-auxiliary-proc">auxiliary process</glossterm>