[Doc] Document latest Mcore toggle moe_hybridep_pad_uneven_dispatch_inputs for HybridEP THD training

[zhongbozhu]

What does this PR do ?

Mcore dependency PR should be merged first:

Mcore dev: NVIDIA/Megatron-LM#5048

Mcore main:

• [Dev] add support for deepep/hybridep dispatcher under thd format training NVIDIA/Megatron-LM#4816
• [main] add thd sequence packing dispatcher support for main NVIDIA/Megatron-LM#5008

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• Add specific line by line info of high level changes in this PR.

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