Clean up some elog messages and comments for do_pg_stop_backup and do_pg_start_backup

Hi all,

Alvaro has cleaned up a couple of error messages recently so as they do
not include the function name in what gets translated as per 68f6f2b7.

While looking in the code for similar patterns, I have been reminded
that pg_stop_backup() is included in some messages when waiting for
segments to be archived. This has resulted in an exchange between Tom
and me here:
https://www.postgresql.org/message-id/E1gZg2W-0008Lq-0w@gemulon.postgresql.org

The thing is that the current messages are actually misleading, because
for base backups taken by the replication protocol pg_stop_backup is
never called, which is I think confusing. While looking around I have
also noticed that the top comments of do_pg_start_backup and
do_pg_stop_backup also that they are used with BASE_BACKUP.

Attached is a patch to reduce the confusion and improve the related
comments and messages.

Thoughts?
--
Michael