[docs] consolidate diffusion execution modes (#5599)

Signed-off-by: bjf-frz <frz123db@gmail.com>

docs/contributing/model/adding_diffusion_model.md

@@ -755,7 +755,8 @@ omni = Omni(model="your-model", ulysses_degree=2, ring_degree=2)
 
 ### Step Execution
 
-See detailed design guide: [How to add step execution support](../../design/feature/diffusion_step_execution.md)
+See the detailed
+[Diffusion Continuous Batching design guide](../../design/feature/diffusion_continuous_batching.md).
 
 Use this only when your pipeline can be split into stable request-scoped and
 step-scoped phases. The reference implementation is
@@ -769,9 +770,8 @@ step-scoped phases. The reference implementation is
 Do not enable `step_execution=True` until those four methods are implemented
 and validated against the request-level path.
 
-If you want the pipeline to work with the experimental batched step-wise path
-(`max_num_seqs > 1`), also see:
-[Continuous Batching for Step-Wise Diffusion](../../design/feature/diffusion_continuous_batching.md).
+The same design guide covers the experimental batched step-wise path used when
+`max_num_seqs > 1`.
 
 If you expose this in example scripts or recipes, keep it opt-in. Surface
 runtime features like `step_execution` as optional flags instead of silently