PlatformMutex docs update

Author: fkjagodzinski

docs/api/platform/PlatformMutex.md

@@ -2,24 +2,31 @@
 
 <span class="images">![](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_platform_mutex.png)<span>PlatformMutex class hierarchy</span></span>
 
-The PlarformMutex class provides mutex stub functions in the absence of RTOS. This class enables you to use driver/application code when the RTOS is not present.
+The PlatformMutex class is used to synchronize the execution of threads.
+
+The PlatformMutex class is used by the Mbed OS drivers instead of [Mutex](mutex.html).
+This enables the use of drivers when the Mbed OS is compiled without the RTOS.
+For example, please see [AnalogIn](analogin.html), [BusOut](busout.html),
+[SPI](spi.html), [Serial](serial.html) and [I2C](i2c.html).
+
+<span class="notes">**Note:**
+For the standard use of RTOS mutexes, please see [Mutex](mutex.html).</span>
 
 ### PlatformMutex class reference
 
 [![View code](https://www.mbed.com/embed/?type=library)](http://os-doc-builder.test.mbed.com/docs/development/mbed-os-api-doxy/class_platform_mutex.html)
 
 ### PlatformMutex example
 
-The code below demonstrates usage of PlatformMutex.
+PlatformMutex usage example for Mbed OS compiled with the RTOS.
 
 [![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed_example/code/mbed-os-example-platform-mutex/)](https://os.mbed.com/teams/mbed_example/code/mbed-os-example-platform-mutex/file/2084d9e90526/main.cpp)
 
-Mbed OS uses the PlatformMutex class instead of the RTOS mutex for all drivers. For example, please see [AnalogIn](analogin.html), [BusOut](busout.html), [SPI](spi.html), [Serial](serial.html) and [I2C](i2c.html).
-
 ### Related content
 
-- [AnalogIn](analogin.html).
-- [BusOut](busout.html).
-- [SPI](spi.html).
-- [Serial](serial.html).
+- [Mutex](mutex.html),
+- [AnalogIn](analogin.html),
+- [BusOut](busout.html),
+- [SPI](spi.html),
+- [Serial](serial.html),
 - [I2C](i2c.html).