man/systemd.exec: MountFlags=shared behaviour was changed (fixed?)

man/systemd.exec: MountFlags=shared behaviour was changed (fixed?)

The behaviour described *was* observed on Fedora 28
(systemd-238-9.git0e0aa59), with and without SELinux.  I don't actually
know why though!  It contradicts my understanding of the code, including an
explicit comment in the code.

Testing in a VM upgraded to v239-792-g1327f272d, this behaviour goes away.

Test case:

# /etc/systemd/system/mount-test.service
[Service]
MountFlags=shared
Type=oneshot
ExecStart=/usr/bin/ls -l /proc/1/ns/mnt /proc/self/ns/mnt
ExecStart=/usr/bin/grep ext4 /proc/self/mountinfo

Weird old behaviour: new mount namespace but / is fully shared.

lrwxrwxrwx. 1 root root 0 Sep 14 11:18 /proc/1/ns/mnt -> mnt:[4026531840]
lrwxrwxrwx. 1 root root 0 Sep 14 11:48 /proc/self/ns/mnt ->
mnt:[4026532851]

968 967 253:0 / / rw,relatime shared:1 - ext4 /dev/mapper/alan_dell_2016...

Current behaviour: / is not fully shared

lrwxrwxrwx. 1 root root 0 Sep 14 11:39 /proc/1/ns/mnt -> mnt:[4026531840]
lrwxrwxrwx. 1 root root 0 Sep 14 11:41 /proc/self/ns/mnt ->
mnt:[4026532329]

591 558 8:3 / / rw,relatime shared:313 master:1 - ext4 /dev/sda3 rw,secl...