Update overclocking.md (#975)

* Update overclocking.md

* Update overclocking.md

* copy edits

configuration/config-txt/overclocking.md

@@ -62,16 +62,18 @@ On the Pi 2/Pi 3, setting this flag will disable the GPU from moving into turbo
 
 The GPU core, CPU, SDRAM and GPU each have their own PLLs and can have unrelated frequencies. The h264, v3d and ISP blocks share a PLL. For more information [see here](https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=6201&start=275#p168042).
 
-To view the Pi's current frequency, type: `cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq`. Divide the result by 1000 to find the value in MHz.
+To view the Pi's current frequency, type: `cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq`. Divide the result by 1000 to find the value in MHz. Note that this frequency is the kernel *requested* frequency, and it is possible that any throttling (for example at high temperatures) may mean the CPU is actually running more slowly than reported. An instantaneous measurement of the actual ARM CPU frequency can be retrieved using the vcgencmd `vcgencmd measure_clock arm`. This is displayed in Hertz.
 
 ## Monitoring core temperature
 
-To view the Pi's temperature, type: `cat /sys/class/thermal/thermal_zone0/temp`. Divide the result by 1000 to find the value in Celsius.
+To view the Pi's temperature, type `cat /sys/class/thermal/thermal_zone0/temp`. Divide the result by 1000 to find the value in degrees Celsius. Alternatively, there is a vcgencmd, `vcgencmd measure_temp` that interrogates the GPU directly for its temperature.
 
 Whilst hitting the temperature limit is not harmful to the SoC, it will cause CPU throttling. A heatsink can help to control the core temperature and therefore performance. This is especially useful if the Pi is running inside a case. Airflow over the heatsink will make cooling more efficient. A suitable heatsink is the self-adhesive BGA (ball-grid-array) 14x14x10 mm heatsink available from [RS Components](http://uk.rs-online.com/web/p/heatsinks/6744756/).
 
 With firmware from 12th September 2016 or later, when the core temperature is between 80'C and 85'C, a warning icon showing a red half-filled thermometer will be displayed, and the ARM cores will be throttled back. If the temperature exceeds 85'C, an icon showing a fully-filled thermometer will be displayed, and both the ARM cores and the GPU will be throttled back.
 
+For the Raspberry Pi 3 Model B+, the PCB technology has been changed to provide better heat dissipation and increased thermal mass. In addition, a soft temperature limit has been introduced, with the goal of maximising the time for which a device can "sprint" before reaching the hard limit at 85°C. When the soft limit is reached, the clock speed is reduced from 1.4GHz to 1.2GHz, and the operating voltage is reduced slightly. This reduces the rate of temperature increase: we trade a short period at 1.4GHz for a longer period at 1.2GHz. By default, the soft limit is 60°C, and this can be changed via the temp_soft_limit setting in config.txt.
+
 See the page on [warning icons](../warning-icons.md) for more details.
 
 ## Monitoring voltage
@@ -80,7 +82,7 @@ It is essential to keep the supply voltage above 4.8V for reliable performance.
 
 To monitor the Pi's PSU voltage, you will need to use a multimeter to measure between the VCC and GND pins on the GPIO. More information is available in [power](../../hardware/raspberrypi/power/README.md).
 
-If the voltage drops below 4.63v (+-5%), recent versions of the firmware will show a yellow lightning bolt symbol on the display to indicate a lack of power.
+If the voltage drops below 4.63v (+-5%), recent versions of the firmware will show a yellow lightning bolt symbol on the display to indicate a lack of power, and a message indicating the low voltage state will be added to the kernel log.
 
 See the page on [warning icons](../warning-icons.md) for more details.