Add comment about r28083

This commit was SVN r28084.

The following SVN revision numbers were found above:
  r28083 --> open-mpi/ompi@5411e28c00

ompi/mca/btl/openib/btl_openib_component.c

@@ -2573,6 +2573,10 @@ btl_openib_component_init(int *num_btl_modules,
     init_data->order = mca_btl_openib_component.rdma_qp;
     init_data->list = &mca_btl_openib_component.send_user_free;
 
+    /* Align fragments on 8-byte boundaries (instead of 2) to fix bus errors that
+       occur on some 32-bit platforms. Depending on the size of the fragment this
+       will waste 2-6 bytes of space per frag. In most cases this shouldn't waste
+       any space. */
     if (OMPI_SUCCESS != ompi_free_list_init_ex_new(
                 &mca_btl_openib_component.send_user_free,
                 sizeof(mca_btl_openib_put_frag_t), 8,