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