Add description of system_reset to power management page

Author: bulislaw

docs/reference/api/platform/PowerManagement.md

@@ -56,6 +56,16 @@ void main()
 }
 ```
 
+### System reset
+
+Mbed OS provides a standardized call to power cycle the system:
+
+```c++
+void system_reset();
+```
+
+After the call the processor and most components will reset, but it will not affect the debug subsystem.
+
 ### Function reference
 
 [![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/v5.7/mbed-os-api-doxy/mbed__sleep_8h_source.html)