RUM: document account.* attributes for the mobile SDKs

[qsellem]

What does this PR do? What is the motivation?

The SDK-level account APIs (setAccountInfo / addAccountExtraInfo / clearAccountInfo) shipped in the mobile SDKs in June 2025, but account.* is only documented for the Browser SDK. Customers on mobile currently have no way to discover the feature — the only mention anywhere in the English docs is an incidental mock-SDK snippet on the Expo page.

This PR adds a ### Track user accounts section next to the existing ### Track user sessions section in Advanced Configuration, mirroring the Browser page's ## Account section, for every platform whose SDK actually exposes the API:

Platform	API	Since
iOS	Datadog.setAccountInfo(id:name:extraInfo:)	2.29.0
Android	Datadog.setAccountInfo(id, name, extraInfo)	2.23.0
Flutter	DatadogSdk.instance.setAccountInfo(...)	2.12.0
React Native	DdSdkReactNative.setAccountInfo({...})	3.0.0
Kotlin Multiplatform	Datadog.setAccountInfo(id, name, extraInfo)	1.2.0

Each section documents the account.id / account.name attributes, the set → append → clear lifecycle, that extraInfo keys are reported under account.<key>, that addAccountExtraInfo before setAccountInfo is a no-op, and that clearing empties account on the active session and view (stop them first to retain it).

Two related fixes in the same area:

• MAUI already documented the API. Added the missing attribute table and renamed the heading from Track account sessions to Track user accounts for consistency with the other platforms.
• C++ documented the attributes as acc.id / acc.name, but the SDK serializes an account object (logging.hpp#L42, crash_handling.cpp#L118), so the queryable attributes are account.id / account.name. A customer following the current page would query on an attribute that does not exist.

All API names, signatures, versions, and behavior notes were verified against the SDK source, not inferred from the Browser docs.

Merge readiness

• Ready for merge

For Datadog employees:

AI assistance

Used Claude Code to cross-check each SDK's source for API availability, exact signatures, and introduction versions, and to draft the sections. Reviewed manually.

Additional notes

Deliberately out of scope, but worth flagging for the owning teams:

• Unity documents Track user sessions but the Unity SDK exposes no account API at all (only SetUserInfo). That's an SDK gap, not a docs one. Roku likewise has no account API.
• tracing/guide/users-accounts.md says user and account info can be propagated from "browser or mobile" applications, then gives only the browser APIs.
• product_analytics/profiles/_index.md points only at datadogRum.setAccount, so mobile-only customers can't tell Account Profiles apply to them.
• The mobile data_collected pages don't list account.* among collected attributes.

Happy to fold any of these in if reviewers prefer them here rather than as follow-ups.

[domalessi]

Created DOCS-15499 for editorial review.