docs: update physical memory documentation by adding N_GENERIC_INITIATOR to enum...

docs: update physical memory documentation by adding N_GENERIC_INITIATOR to enum node_states

While reading physical_memory.rst, I noticed that N_GENERIC_INITIATOR has not been update
from the node_states list, even though it's already added in commit 894c26a1c274b8eafbb4b1dad67e70e51a106061.

Signed-off-by: Kim Tae Hyun <kimth0312@gmail.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250802125801.10068-1-kimth0312@gmail.com