[DOC] Clarify input and output field description on text_chunking processor

[yuye-aws]

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As is suggested by a github user, we can better clarify the parameter description on input and output in text_chunking processor opensearch-project/neural-search#895 (comment).

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

Since 2.13.

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Issue in neural-search: opensearch-project/neural-search#895
Existing document: https://opensearch.org/docs/latest/ingest-pipelines/processors/text-chunking/