include/nuttx/lcd: change meaning of buffer argument for putarea

The putarea documentation originally suggested that provided buffer
contains just the rectangle to be updated.
The commit 664d45dcba changed the expected
behavior for lcd_framebuffer but failed to propagate this change to the
driver's documentation. Now the expected behavior is that the whole
frame is passed in buffer and it is driver's responsibility to pick the
correct pixels according to the provided rectangle coordinates.

This change requires update of the LCD drivers if they implement this
function.

include/nuttx/lcd/lcd.h

@@ -79,7 +79,9 @@ struct lcd_planeinfo_s
    *  col_start - Starting column to write to (range: 0 <= col <= xres)
    *  col_end   - Ending column to write to
    *              (range: col_start <= col_end < xres)
-   *  buffer    - The buffer containing the area to be written to the LCD
+   *  buffer    - The buffer containing the complete frame to be written to
+   *              the display (the correct rows and columns have to be
+   *              selected from it)
    *
    * NOTE: this operation may not be supported by the device, in which case
    * the callback pointer will be NULL. In that case, putrun() should be