message: Add description of merge segments to docs
diff: @@ -237,3 +237,5 @@ In this example we will collect the arithmetic ops in a TensorRT segment and the
  return ()
  In some cases this approach may create adjacent segments in the partition which have the same target. As a clean-up step we can consolidate these adjacent segments to further reduce the number of segments in the final partition.
  +The merge segments step identifies a list of segments that are adjacent in the graph, have the same target, and are not marked as `do_not_merge`. The nodes from these segments will be combined into a single new segment that will replace the merged segments in the partition.
  +The `do_not_merge` marking is used to prevent merging of segments created for conditional nodes and loops that are handled as special cases in graph stitching and should not be merged with adjacent segments of the same type.
  