cpuidle / Documentation: Update cpuidle MAINTAINERS entry

Update the MAINTAINERS entry for cpuidle by making it clear that it
is not just drivers and adding a documentation record to it.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

MAINTAINERS

@@ -3964,13 +3964,14 @@ S:	Supported
 F:	drivers/cpuidle/cpuidle-exynos.c
 F:	arch/arm/mach-exynos/pm.c
 
-CPUIDLE DRIVERS
+CPU IDLE TIME MANAGEMENT FRAMEWORK
 M:	"Rafael J. Wysocki" <rjw@rjwysocki.net>
 M:	Daniel Lezcano <daniel.lezcano@linaro.org>
 L:	linux-pm@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git
 B:	https://bugzilla.kernel.org
+F:	Documentation/admin-guide/pm/cpuidle.rst
 F:	drivers/cpuidle/*
 F:	include/linux/cpuidle.h