PM: describe kernel policy regarding wakeup defaults (v. 2)

This patch (as1381b) updates a comment describing the kernel's policy
toward enabling wakeup by default.

It also makes device_set_wakeup_capable() actually do something when
CONFIG_PM isn't enabled.  It's not clear this is necessary; however if
it isn't then device_init_wakeup() and device_can_wakeup() should also
be do-nothing routines.  Furthermore, I don't expect this change to
have any noticeable effect -- but if it does then clearly the old
behavior was wrong.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>