tests: user_property: Clarify comments

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #16248

tests/zfs-tests/tests/functional/cli_root/zpool_set/user_property_002_neg.ksh

@@ -51,13 +51,23 @@ log_onexit cleanup_user_prop $TESTPOOL
 typeset -a names=()
 typeset -a values=()
 
-# Too long property name (256 bytes, which is the 256-byte limit minus 1 byte
-# for the null byte plus 1 byte to reach back over the limit)
-names+=("$(awk 'BEGIN { printf "x:"; while (c++ < (256 - 2 - 1 + 1)) printf "a" }')")
+# A property name that is too long consists of 256 or more bytes (which is (1)
+# the 256-byte limit (2) minus 1 byte for the null byte (3) plus 1 byte to
+# reach back over the limit).
+names+=("$(awk '
+	BEGIN {
+		# Print a 2-byte prefix of the name.
+		printf "x:";
+		# Print the remaining 254 bytes.
+		while (c++ < (256 - 2 - 1 + 1))
+			printf "a"
+	}'
+)")
 values+=("too-long-property-name")
-# Too long property value (the limits are 1024 on FreeBSD and 4096 on Linux, so
-# pick the right one; the too long value is, e.g., the limit minus 1 bytes for the
-# null byte plus 1 byte to reach back over the limit)
+# A property value that is too long consists of at least 1024 bytes on FreeBSD
+# and 4096 bytes on Linux.
+# The smallest too-long value is (1) the limit (2) minus 1 byte for the null
+# byte (2) plus 1 byte to reach back over the limit).
 if is_linux; then
 	typeset ZFS_MAXPROPLEN=4096
 else