workqueue: rename worker_pool->assoc_mutex to ->manager_mutex

Manager operations are currently governed by two mutexes -
pool->manager_arb and ->assoc_mutex.  The former is used to decide who
gets to be the manager and the latter to exclude the actual manager
operations including creation and destruction of workers.  Anyone who
grabs ->manager_arb must perform manager role; otherwise, the pool
might stall.

Grabbing ->assoc_mutex blocks everyone else from performing manager
operations but doesn't require the holder to perform manager duties as
it's merely blocking manager operations without becoming the manager.

Because the blocking was necessary when [dis]associating per-cpu
workqueues during CPU hotplug events, the latter was named
assoc_mutex.  The mutex is scheduled to be used for other purposes, so
this patch gives it a more fitting generic name - manager_mutex - and
updates / adds comments to explain synchronization around the manager
role and operations.

This patch is pure rename / doc update.

Signed-off-by: Tejun Heo <tj@kernel.org>