Improve comments and logs in do_pg_stop/start_backup

The function name pg_stop_backup() has been included for ages in some
log messages when stopping the backup, which is confusing for base
backups taken with the replication protocol because this function is
never called.  Some other comments and messages in this area are
improved while on it.

The new wording is based on input and suggestions from several people,
all listed below.

Author: Michael Paquier
Reviewed-by: Peter Eisentraut, Álvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/20181221040510.GA12599@paquier.xyz
