feat(Infra Flex): Improved the NR Flex doc

[nbaenam]

Improved the NR Flex doc.

Ticket: NR-324250

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[github-actions]

[!NOTE]

Our Vale automated writing feedback tool has suggestions for this PR. You can view these suggestions in the Files changed tab. If you're not a New Relic docs writer, we'll review any comments and you don't need to take any action (though feel free to make edits if you like!).