Improve wxThread::SetPriority() comment and documentation.

Don't claim that SetPriority() can only be called before the thread is
created, this is just wrong.

Do document that it must be done after creating the thread in wxMSW (this is a
limitation of this port as POSIX implementation allows calling it at any
moment and could be lifted in the future).

Closes #16809.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78418 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/thread.h

@@ -1258,6 +1258,11 @@ public:
           - @c wxPRIORITY_MIN: 0
           - @c wxPRIORITY_DEFAULT: 50
           - @c wxPRIORITY_MAX: 100
+
+        Notice that in the MSW implementation the thread priority can currently
+        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.
     */
     void SetPriority(unsigned int priority);