workqueue: Update documentation as per system_percpu_wq naming

workqueue: Update documentation as per system_percpu_wq naming

Update documentation to use "per-CPU workqueue" instead of
"global workqueue" to match the system_wq to system_percpu_wq
rename. The workqueue behavior remains unchanged; this just
aligns terminology with the clearer naming.

Fixes: a2be943b46b4 ("workqueue: replace use of system_wq with system_percpu_wq")
Signed-off-by: Mallesh Koujalagi <mallesh.koujalagi@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>