[DOC] Clarify that text embedding model will do truncation

[ylwu-amzn]

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[ylwu-amzn]

Read more details opensearch-project/ml-commons#2466

the trouble here is not whether it's configurable its about user understanding. If someone is new to vector search this is a really easy way to shoot yourself in the foot and never know what went wrong. I've talked to several people already who have completely abandoned vector search because they thought the relevancy was bad. Turns out their documents were just truncated because they didn't understand how vectorization worked.

[kolchfa-aws]

@dtaivpp Could you raise a PR for this since you are familiar with this issue?