[UT-2091] Update Segment "Messaging Use" Docs

[diana-smirnova]

Why are you making this change? (required)

Clarifying that the segment "Messaging Use" section only include currently enabled campaigns / canvases. This came up in a PQ: https://servicedesk.braze.com/browse/PQ-5204
Also separating out the "Messaging use" and "Historical membership" sections since they're unrelated.

Related PRs, issues, or features (optional)

Ticket: UT-2091
PR that introduced this change: https://github.com/Appboy/platform/pull/55394

Feature release date (optional)

• N/A

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[diana-smirnova]

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[rachel-feinberg]

Lgtm