doc/cephfs: Squid and later - subvolume quiesce

doc/cephfs: Squid and later - subvolume quiesce

Add a note to the "Subvolume quiesce" section that says that the
information in the section applies only to the Squid and later releases
of Ceph. This is included here so that I don't overwrite the Reef and
Quincy documentation with irrelevant information, and so that I don't
overwrite the Squid information with blank space where the "Subvolume
quiesce" section should be.

Signed-off-by: Zac Dover <zac.dover@proton.me>