Fix wxThread::SetPriority() documentation

The documentation's notes about MSW limitations about setting priority
before creating the thread do not appear to be true (anymore). Thread
priority is already set by Create() if SetPriority() was called earlier.
Setting it immediately just failed, because the thread did not exist
yet, but this was fixed by the previous commit.

include/wx/thread.h

@@ -559,9 +559,6 @@ public:
     // priority
         // Sets the priority to "prio" which must be in 0..100 range (see
         // also wxPRIORITY_XXX constants).
-        //
-        // NB: under MSW the priority can only be set after the thread is
-        //     created (but possibly before it is launched)
     void SetPriority(unsigned int prio);
 
         // Get the current priority.