[doc] refactor: add constraints on the use of vpp and mbridge parameters

[zjchenn]

What does this PR do?

This PR updates the Ascend backend documentation to clarify a current compatibility constraint: mbridge does not support VPP (virtual_pipeline_model_parallel_size) in our current stack.

Add a note in docs/ascend_tutorial/features/ascend_backend_features.md (line 276) that VPP should be used only when mbridge is disabled.
Clarify that because verl now enables mbridge by default, users who enable VPP must explicitly set actor_rollout_ref.actor.megatron.use_mbridge=False.
This avoids confusing runtime failures in Megatron actor log-prob computation when using VPP with mbridge.

#4528 (use_mbridge default enabled)

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[gemini-code-assist]

Code Review

This pull request updates the ascend_backend_features.md documentation to include a note about the incompatibility between mbridge and VPP (virtual_pipeline_model_parallel_size) and provides instructions for their configuration. The feedback suggests improving the clarity and accuracy of this note by using full parameter paths instead of abbreviations to prevent configuration errors.