Document that wxThread::SetPriority() is broken under Unix

The code setting thread priority doesn't work without changing the
scheduling policy as thread priorities are simply ignored when using the
default SCHED_OTHER (at least under Linux and NetBSD, but probably other
systems too).

See #18195.

interface/wx/thread.h

@@ -1248,6 +1248,9 @@ public:
         be only set after creating the thread with CreateThread(). But under
         all platforms this method can be called either before launching the
         thread using Run() or after doing it.
+
+        Please note that currently this function is not implemented when using
+        the default (@c SCHED_OTHER) scheduling policy under POSIX systems.
     */
     void SetPriority(unsigned int priority);
 

src/unix/threadpsx.cpp

@@ -1005,6 +1005,12 @@ static bool SetThreadPriority(pthread_attr_t& attr, int prio)
         return false;
     }
 
+    // TODO: on most (all?) systems, thread priorities can't be used with
+    //       SCHED_OTHER policy, so we need to check if this is the current
+    //       policy and change it to something else (SCHED_FIFO or SCHED_RR?)
+    //       in order to be able to actually change the priority as without
+    //       doing it the code below just not going to work.
+
 #ifdef __VMS__
    /* the pthread.h contains too many spaces. This is a work-around */
 # undef sched_get_priority_max