ACPI: Better describe ACPI_DEBUGGER

Hi,

For a brief moment I was tricked into thinking that:

  In-kernel debugger (EXPERIMENTAL) (ACPI_DEBUGGER) [N/y/?] (NEW)

might be something useful. Better describe the feature to reduce
such confusion.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>