aux/tc: update docs to indicate replaced buffers have multiple pipe_resources

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30040>

src/gallium/auxiliary/util/u_threaded_context.h

@@ -152,6 +152,8 @@
  *    which must be managed by the driver; if a buffer is bound multiple times in
  *    the same binding point (e.g., vertex buffer slots 0,1,2), this will be counted
  *    as a single rebind.
+ *    A buffer which has had its backing storage replaced may have its backing storage
+ *    accessed through multiple pipe_resources.
  *
  *
  * Optional resource busy callbacks for better performance