doc: nvme-cuse is no longer experimental

This has been actively supported for several years
now, but this part of the documentation was never
updated to reflect that.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Iad69552ecd37a01ca6d281a0169d527c59c233b3
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/16582


Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: John Kariuki <John.K.Kariuki@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>