doc: add systemd unit part for FUSE mounts in fstab doc
To make FUSE-mounted CephFS persist across reboots, user also needs to
start and enable the systemd units. Add that part to the document for
fstab, instead of mentioning it in "Mount CephFS using FUSE" doc. Also,
wrap few lines and rename mountpoint to /mnt/mycephfs in examples to
keep them same across docs.
Fixes: https://tracker.ceph.com/issues/42298 Signed-off-by: Rishabh Dave <ridave@redhat.com>