Improve warning message in pg_signal_backend()

Previously, invoking pg_terminate_backend() or pg_cancel_backend()
with the postmaster PID produced a "PID XXXX is not a PostgresSQL
server process" warning, which does not make sense. Change to
"backend process" to make the message more exact.

Nathan Bossart, based on an idea from Bharath Rupireddy with
input from Tom Lane and Euler Taveira

Discussion: https://www.postgresql.org/message-id/flat/CALj2ACW7Rr-R7mBcBQiXWPp=JV5chajjTdudLiF5YcpW-BmHhg@mail.gmail.com

src/backend/storage/ipc/signalfuncs.c

@@ -57,6 +57,10 @@ pg_signal_backend(int pid, int sig)
 	 * arbitrary process to prevent that. But since so far all the callers of
 	 * this mechanism involve some request for ending the process anyway, that
 	 * it might end on its own first is not a problem.
+	 *
+	 * Note that proc will also be NULL if the pid refers to an auxiliary
+	 * process or the postmaster (neither of which can be signaled via
+	 * pg_signal_backend()).
 	 */
 	if (proc == NULL)
 	{
@@ -65,7 +69,8 @@ pg_signal_backend(int pid, int sig)
 		 * if one backend terminated on its own during the run.
 		 */
 		ereport(WARNING,
-				(errmsg("PID %d is not a PostgreSQL server process", pid)));
+				(errmsg("PID %d is not a PostgreSQL backend process", pid)));
+
 		return SIGNAL_BACKEND_ERROR;
 	}