Update content in PlatformMutex.md

Clarify description, as in PR #806

Author: Amanda Butler

docs/api/platform/PlatformMutex.md

@@ -2,24 +2,25 @@
 
 <span class="images">![](https://os.mbed.com/docs/v5.10/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.
+You can use the PlatformMutex class to synchronize the execution of threads.
+
+The Mbed OS drivers use the PlatformMutex class instead of [Mutex](mutex.html). This enables the use of drivers when the Mbed OS is compiled without the RTOS. For examples, please see [AnalogIn](analogin.html), [BusOut](busout.html), [Serial](serial.html), [SPI](spi.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)](https://os.mbed.com/docs/v5.10/mbed-os-api-doxy/class_platform_mutex.html)
 
 ### PlatformMutex example
 
-The code below demonstrates usage of PlatformMutex.
-
 [![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
 
+- [Mutex](mutex.html).
 - [AnalogIn](analogin.html).
 - [BusOut](busout.html).
-- [SPI](spi.html).
 - [Serial](serial.html).
+- [SPI](spi.html).
 - [I2C](i2c.html).