revise docs for _inplace

We call vips_image_inplace() automatically now, so there's no need for
clients to use this.

see https://github.com/libvips/libvips/issues/1610

cplusplus/VImage.cpp

@@ -413,7 +413,8 @@ VOption::set_operation( VipsOperation *operation )
 #ifdef VIPS_DEBUG_VERBOSE
 			printf( "set_operation: " );
 			vips_object_print_name( VIPS_OBJECT( operation ) );
-			char *str_value = g_strdup_value_contents( &(*i)->value );
+			char *str_value =
+				g_strdup_value_contents( &(*i)->value );
 			printf( ".%s = %s\n", (*i)->name, str_value );
 			g_free( str_value );
 #endif /*VIPS_DEBUG_VERBOSE*/

libvips/iofuncs/image.c

@@ -3535,6 +3535,9 @@ vips__image_wio_output( VipsImage *image )
  * After calling this function you can both read and write the image with 
  * VIPS_IMAGE_ADDR().
  *
+ * This method is called for you by the base class of the draw operations, 
+ * there's no need to call it yourself.
+ *
  * Since this function modifies @image, it is not thread-safe. Only call it on
  * images which you are sure have not been shared with another thread. 
  * All in-place operations are inherently not thread-safe, so you need to take