[8.x](backport #42114) [Filebeat] minor cleanup to aws-s3 input docs (#…

…42122)

Minor improvements to the Filebeat aws-s3 input documentation.
Wrap long lines to generally limit lines to 80 characters.
Increase the heading level for the csv and parquet decoders so
that they are sub-headings of the decoder section.
Fix several grammatical issues and rewrite some unclear sections.

(cherry picked from commit ef691b6)

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Co-authored-by: Andrew Kroh <andrew.kroh@elastic.co>