VIDEO: better document the correct use of CONFIG_FB_ADDR

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Anatolij Gustschin <agust@denx.de>

README

@@ -2714,10 +2714,13 @@ FIT uImage format:
 		CONFIG_FB_ADDR
 
 		Define CONFIG_FB_ADDR if you want to use specific
-		address for frame buffer.
+		address for frame buffer.  This is typically the case
-		Then system will reserve the frame buffer address to
+		when using a graphics controller has separate video
-		defined address instead of lcd_setmem (this function
+		memory.  U-Boot will then place the frame buffer at
-		grabs the memory for frame buffer by panel's size).
+		the given address instead of dynamically reserving it
+		in system RAM by calling lcd_setmem(), which grabs
+		the memory for the frame buffer depending on the
+		configured panel size.
 
 		Please see board_init_f function.