[8.8] [DOCS] Adds section about tokens to ELSER conceptual (backport #2568)

[mergify]

This is an automatic backport of pull request #2568 done by Mergify.
Cherry-pick of f9c8a20 has failed:

On branch mergify/bp/8.8/pr-2568
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	both modified:   docs/en/stack/ml/nlp/ml-nlp-elser.asciidoc

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