BD-3906, BD-3839: Clarify email identifier and prioritization value

[lydia-xie]

Why are you making this change? (required)

Clarify email identifier and prioritization value for:

• POST: Send Campaigns Using API-Triggered Delivery
• POST: Send Canvas Messages Using API-Triggered Delivery

Related PRs, issues, or features (optional)

• BD-3906
• BD-3839

Feature release date (optional)

• N/A

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[internetisaiah]

LGTM, left one comment