Doxygen comment fix for Thread class

Author: maciejbocianski

rtos/Thread.h

@@ -246,7 +246,7 @@ class Thread : private mbed::NonCopyable<Thread> {
 
     /** Set the specified Thread Flags for the thread.
       @param   signals  specifies the signal flags of the thread that should be set.
-      @return  previous signal flags of the specified thread or osFlagsError in case of incorrect parameters.
+      @return  signal flags after setting or osFlagsError in case of incorrect parameters.
     */
     int32_t signal_set(int32_t signals);
 
@@ -305,7 +305,7 @@ class Thread : private mbed::NonCopyable<Thread> {
 
     /** Clears the specified Thread Flags of the currently running thread.
       @param   signals  specifies the signal flags of the thread that should be cleared.
-      @return  resultant signal flags of the specified thread or osFlagsError in case of incorrect parameters.
+      @return  signal flags before clearing or osFlagsError in case of incorrect parameters.
     */
     static int32_t signal_clr(int32_t signals);