Updated /campaigns/trigger/send docs to include newly supported attachments feature

[andriibrz]

/campaigns/trigger/send support attachments in the same way as /messages/send for emails. The support was rolled out today, we are ready to update public docs.

Pull Request/Issue Resolution

Description of Change:

I'm changing https://www.braze.com/docs/api/endpoints/messaging/send_messages/post_send_triggered_campaigns because attachments support is now available to all Braze Customers

Is this change associated with a Braze feature/product release?

• Yes (Insert Feature Release Date Here)
• [ ✔️] No

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[lydia-xie]

lgtm!