docs(native): document how to set attribute on spans/transactions

[JoshuaMoelans]

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Closes getsentry/sentry-native#1112

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Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 27, 2025 11:05am

2 Skipped Deployments

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[codecov]

Bundle Report

Changes will increase total bundle size by 195 bytes (0.0%) ⬆️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	10.62MB	201 bytes (0.0%) ⬆️
sentry-docs-client-array-push	9.3MB	6 bytes (-0.0%) ⬇️