[doc] Fix audit opaque_id description

[pgomulka]

opaque_id did not mention that opaque_id has a semantic and is used for
throttling deprecation/slow logs

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[pgomulka]

ok to test

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[pgomulka]

closing as the doc change was included in #82849