PR #21826: merge_repeated option is confusing

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Imported from GitHub PR #21826

I have the same question with [WIP: Remove invalid merge_repeated option from CTC beam decoder](#15586), it's a pity I haven't seen any changes for so long.
Generally I will use the default value of merge_repeated: True, but I found it's confusing, that is, I got the wrong anser, it has been explained well in [WIP: Remove invalid merge_repeated option from CTC beam decoder](#15586).
And the top path in ctc_beam_search_decoder is similar with sequence in ctc_greedy_decoder, this is confusing, I have found the project [CRNN](https://github.com/Belval/CRNN/blob/master/CRNN/crnn.py)(line 167) and some other projects use the wrong settings.
So I think it's better to give a explain here, this has no conflict with the existing code.

Copybara import of the project:

  - e357bcea merge_repeated option is confusing by Dao Zhang <zhangdao@buaa.edu.cn>
  - a0467d35cc19293fa16918658a7f98e18ead7f87 Merge e357bcea into 34ef4... by Dao Zhang(??) <zhangdao@buaa.edu.cn>

PiperOrigin-RevId: 212466200