man: Correct information on sysext masking

While I had tested that a symlink to /dev/null works to "mask" a sysext
I must have gotten something wrong and thus the instructions in
519c2f0d6b don't work. What works,
at least at the moment, is to instead have an empty directory with the
extension name under /etc/extensions/.
Correct the info in the man page and add a test for it.

test/units/testsuite-50.sh

@@ -356,6 +356,19 @@ systemctl is-active testservice-50f.service
 systemd-dissect --umount "${image_dir}/app0"
 systemd-dissect --umount "${image_dir}/app1"
 
+# Test that an extension consisting of an empty directory under /etc/extensions/ takes precedence
+mkdir -p /var/lib/extensions/
+ln -s /usr/share/app-nodistro.raw /var/lib/extensions/app-nodistro.raw
+systemd-sysext merge
+grep -q -F "MARKER=1" /usr/lib/systemd/system/some_file
+systemd-sysext unmerge
+mkdir -p /etc/extensions/app-nodistro
+systemd-sysext merge
+test ! -e /usr/lib/systemd/system/some_file
+systemd-sysext unmerge
+rmdir /etc/extensions/app-nodistro
+rm /var/lib/extensions/app-nodistro.raw
+
 echo OK >/testok
 
 exit 0