Documentation: cpufreq / boost: Update BOOST documentation

Since the support for software and hardware controlled boosting has
been added, update the corresponding documentation.

Signed-off-by: Lukasz Majewski <[e-mail]>
Signed-off-by: Myungjoo Ham <[e-mail]>
Acked-by: Viresh Kumar <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/cpu-freq/boost.txt

···
 Some CPUs support a functionality to raise the operating frequency of
 some cores in a multi-core package if certain conditions apply, mostly
 if the whole chip is not fully utilized and below it's intended thermal
-budget. This is done without operating system control by a combination
-of hardware and firmware.
+budget. The decision about boost disable/enable is made either at hardware
+(e.g. x86) or software (e.g ARM).
 On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",
 in technical documentation "Core performance boost". In Linux we use
 the term "boost" for convenience.
···
 User controlled switch
 ----------------------
 
-To allow the user to toggle the boosting functionality, the acpi-cpufreq
-driver exports a sysfs knob to disable it. There is a file:
+To allow the user to toggle the boosting functionality, the cpufreq core
+driver exports a sysfs knob to enable or disable it. There is a file:
 /sys/devices/system/cpu/cpufreq/boost
 which can either read "0" (boosting disabled) or "1" (boosting enabled).
-Reading the file is always supported, even if the processor does not
-support boosting. In this case the file will be read-only and always
-reads as "0". Explicitly changing the permissions and writing to that
-file anyway will return EINVAL.
+The file is exported only when cpufreq driver supports boosting.
+Explicitly changing the permissions and writing to that file anyway will
+return EINVAL.
 
 On supported CPUs one can write either a "0" or a "1" into this file.
 This will either disable the boost functionality on all cores in the
-whole system (0) or will allow the hardware to boost at will (1).
+whole system (0) or will allow the software or hardware to boost at will
+(1).
 
 Writing a "1" does not explicitly boost the system, but just allows the
-CPU (and the firmware) to boost at their discretion. Some implementations
-take external factors like the chip's temperature into account, so
-boosting once does not necessarily mean that it will occur every time
-even using the exact same software setup.
+CPU to boost at their discretion. Some implementations take external
+factors like the chip's temperature into account, so boosting once does
+not necessarily mean that it will occur every time even using the exact
+same software setup.
 
 
 AMD legacy cpb switch