partx: document -d vs. --nr and fix test

The commit ab025087f9 has disabled error
message, but unfortunately it keeps wrong return code. This has been fixed
by commit 53ae7d60cf.

This commit add hit about it to docs and fix regression test too.

Signed-off-by: Karel Zak <kzak@redhat.com>

disk-utils/partx.8

@@ -53,7 +53,10 @@ Add the specified partitions, or read the disk and add all partitions.
 Print the SIZE column in bytes rather than in human-readable format.
 .TP
 .BR \-d , " \-\-delete"
-Delete the specified partitions or all partitions.
+Delete the specified partitions or all partitions.  It is not error to
+remove non-existing partitions, so this option is possible to use together with
+large \fB\-\-nr\fR ranges without care about the current partitions set on
+the device.
 .TP
 .BR \-g , " \-\-noheadings"
 Do not print a header line with \fB\-\-show\fR or \fB\-\-raw\fR.

tests/ts/partx/partx

@@ -137,9 +137,14 @@ udevadm settle
 ts_init_subtest "delete-non-existent"
 #attempt to remove it again
 {
+	# remove non-existing partitions (ENXIO) is not error
+	#
+	# see ab025087f91b66ee8e23a16bc49eb0d9bd421d65 and
+	#     53ae7d60cfeacd4e87bfe6fcc015b58b78ef4555
+	#
 	$TS_CMD_PARTX -d --nr $PARTS $TS_DEVICE &&
-		echo "partx failed: removed non-existing partition" ||
-		echo "partx: OK"
+		echo "partx: OK" ||
+		echo "partx failed: removed non-existing partition"
 } >$TS_OUTPUT 2>&1
 ts_finalize_subtest