[8.15] [DOCS] Adds pre-cleaning recommendation to ELSER docs (backport #2796)

[mergify]

Overview

This PR adds a new section to the ELSER conceptual page that recommends cleaning the input text before generating embeddings to achieve the best results.

Preview

ELSER - Pre-cleaning input text

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This is an automatic backport of pull request #2796 done by Mergify.

[mergify]

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	both modified:   docs/en/stack/ml/nlp/ml-nlp-elser.asciidoc

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