W-15896090 Add DataWeave variables to Message Logging policy doc (1.8)

[chris-campbell9]

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[terminology-check[bot]]

This PR contains some terms that are considered warnings according to the Inclusive Product Language initiative

• gateway/1.8/modules/ROOT/pages/policies-included-message-logging.adoc: master Please, use other terminology for the found warnings. You can follow the suggested replacements described here.