src/buffer.h, src/frame.h: Clarify doc of optimization variables.

src/buffer.h

@@ -847,8 +847,8 @@ struct buffer
   struct region_cache *width_run_cache;
   struct region_cache *bidi_paragraph_cache;
 
-  /* Non-zero means don't use redisplay optimizations for
-     displaying this buffer.  */
+  /* Non-zero means disable redisplay optimizations when rebuilding the glyph
+     matrices (but not when redrawing).  */
   bool_bf prevent_redisplay_optimizations_p : 1;
 
   /* Non-zero whenever the narrowing is changed in this buffer.  */

src/frame.h

@@ -375,7 +375,8 @@ struct frame
      set this directly, use SET_FRAME_ICONIFIED instead.  */
   bool_bf iconified : 1;
 
-  /* True if this frame should be redrawn.  */
+  /* True if this frame should be fully redisplayed.  Disables all
+     optimizations while rebuilding matrices and redrawing.  */
   bool_bf garbaged : 1;
 
   /* False means, if this frame has just one window,