netlink: specs: dpll: drop the reference to DPLL_MODE_DETACHED

The lock-status doc tells userspace that DPLL_LOCK_STATUS_UNLOCKED
can be forced by setting DPLL_A_MODE to DPLL_MODE_DETACHED.
But no such value has been defined so far. enum dpll_mode has only
MANUAL and AUTOMATIC. Let's make sure the doc reflects current
reality.

Reviewed-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260924231211.568213-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>