Feature #22477: multifs: remove multifs experimental warnings
mount.ceph: add "fs" alias for "mds_namespace"
I feel "mds_namespace" is not an intuitive name. Let's keep it for backwards compatibility but introduce a cleaner name, "fs". Convert all documentation to use the new name.
Currently will fix this in ceph mount code, which will add the new "fs=<fs_name>" and translate it to "mds_namespace=" before sending mount options to kernel space. Because changing the kernel code, we still need to support the mds_namespace= option forever to backwards compatibility, which will be a little ugly.