Add documentation on invalid thread functions in irq

Author: geky

core/mbed-rtos/rtos/Thread.h

@@ -60,6 +60,7 @@ class Thread {
 
     /** Wait for thread to terminate
       @return  status code that indicates the execution status of the function.
+      @note not callable from interrupt
     */
     osStatus join();
 
@@ -134,17 +135,20 @@ class Thread {
       @param   signals   wait until all specified signal flags set or 0 for any single signal flag.
       @param   millisec  timeout value or 0 in case of no time-out. (default: osWaitForever).
       @return  event flag information or error code.
+      @note not callable from interrupt
     */
     static osEvent signal_wait(int32_t signals, uint32_t millisec=osWaitForever);
 
     /** Wait for a specified time period in millisec:
       @param   millisec  time delay value
       @return  status code that indicates the execution status of the function.
+      @note not callable from interrupt
     */
     static osStatus wait(uint32_t millisec);
 
     /** Pass control to next thread that is in state READY.
       @return  status code that indicates the execution status of the function.
+      @note not callable from interrupt
     */
     static osStatus yield();