Update index to clarify enabling of setting Merge commit

What does this MR do?

Adding in a small update to clarify Merge method setting of Merge commit that needs to be enabled for merge trains to work, since the other two options do not yet work for merge trains. This is based on a Support ticket that came in where customer was trying to get merge trains to work using the other two options (semi-linear history or FF).

Please feel free to adjust as needed.

Related issues

#34515 (closed) #35628 (closed) #282442 (closed)

(Not sure if we want to also include anything in the documentation to clarify that semi-linear history or FF merge commits are not yet supported?)

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When applicable:

• Update the permissions table.
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