Update USP docs under keepalive connections for better readability

[Morgan-P-04]

Proposed changes

This PR updates the Enable keepalive connections section of the Upstream Settings Policy API to improve the readability for users. The change re-orders where the default value for the keepalive directive is mentioned so that users do not have to switch context as much, to align with a suggestion from the team: #2160 (comment)

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[bjee19]

Just for the sake of the guide, since the default nginx keepalive is 32, can we adjust the guide example so the USP isn't setting the keepalive to 32, but instead perhaps 24?