Fix doxygen warnings

1. src/Fl_add_idle.cxx: add missing parameter docs

2. FL/fl_draw.H: rename 1st parameter of fl_draw_pixmap(...) from
   'data' to 'cdata'.

The 2nd warning appears to be caused by a 'friend' declaration in
  FL/Fl_Graphics_Driver.H, line 98:
    'friend FL_EXPORT int fl_draw_pixmap(const char*const* cdata, ...)'

This 'friend' declaration seems to "confuse" some (newer?) doxygen
versions, definitely doxygen 1.13.2.

FL/fl_draw.H

@@ -1155,13 +1155,13 @@ FL_EXPORT Fl_RGB_Image *fl_capture_window(Fl_Window *win, int x, int y, int w, i
   The image is dithered on 8-bit displays so you won't lose color
   space for programs displaying both images and pixmaps.
 
-  \param[in] data pointer to XPM image data
-  \param[in] x,y  position of top-left corner
-  \param[in] bg   background color
+  \param[in] cdata  pointer to XPM image data
+  \param[in] x,y    position of top-left corner
+  \param[in] bg     background color
 
   \returns 0 if there was any error decoding the XPM data.
 */
-FL_EXPORT int fl_draw_pixmap(const char *const *data, int x, int y, Fl_Color bg = FL_GRAY);
+FL_EXPORT int fl_draw_pixmap(const char *const *cdata, int x, int y, Fl_Color bg = FL_GRAY);
 /**
   Draw XPM image data, with the top-left corner at the given position.
   \see fl_draw_pixmap(const char* const* data, int x, int y, Fl_Color bg)

src/Fl_add_idle.cxx

@@ -68,6 +68,7 @@ static void call_idle() {
   FLTK will not recursively call the idle callback.
 
   \param[in] cb   your idle callback
+  \param[in] data an arbitrary data value provided to your callback
 */
 void Fl::add_idle(Fl_Idle_Handler cb, void* data) {
   idle_cb* p = freelist;