Remove redundant doxygen docs of fl_beep()

Also add a comment to all platform driver implementations.

src/drivers/Cocoa/Fl_Cocoa_Screen_Driver.cxx

@@ -158,11 +158,7 @@ void Fl_Cocoa_Screen_Driver::screen_dpi(float &h, float &v, int n)
 }
 
 
-/**
+// Implements fl_beep(). See documentation in src/fl_ask.cxx.
- Emits a system beep message.
- \param[in] type   The beep type from the \ref Fl_Beep enumeration.
- \note \#include <FL/fl_ask.H>
- */
 void Fl_Cocoa_Screen_Driver::beep(int type) {
   switch (type) {
     case FL_BEEP_DEFAULT :

src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx

@@ -1251,6 +1251,7 @@ void Fl_Wayland_Screen_Driver::screen_dpi(float &h, float &v, int n)
 }
 
 
+// Implements fl_beep(). See documentation in src/fl_ask.cxx.
 void Fl_Wayland_Screen_Driver::beep(int type)
 {
   fprintf(stderr, "\007");

src/drivers/WinAPI/Fl_WinAPI_Screen_Driver.cxx

@@ -231,6 +231,7 @@ int Fl_WinAPI_Screen_Driver::w()
 }
 
 
+// Implements fl_beep(). See documentation in src/fl_ask.cxx.
 void Fl_WinAPI_Screen_Driver::beep(int type)
 {
   switch (type) {

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -394,6 +394,7 @@ void Fl_X11_Screen_Driver::screen_dpi(float &h, float &v, int n)
 }
 
 
+// Implements fl_beep(). See documentation in src/fl_ask.cxx.
 void Fl_X11_Screen_Driver::beep(int type)
 {
   int vol;