[DAL-834] Document OAuth scope restrictions

[imaique]

What does this PR do? What is the motivation?

Fixes DAL-834

Updates the Mobile and Third-Party Access documentation to:

• Describe how Application Scope Management handles future scopes.
• Clarify support for the Datadog Mobile App and MCP applications.
• Document required Mobile scopes and authorization revocation behavior.

Merge readiness

• Ready for merge

AI assistance

Used Codex for source research, initial drafting, and style checks. The final wording was validated against shipped behavior.

Additional notes

Validation:

• git diff --check passed.
• Added-content checks found no em dashes, restricted style terms, or internal references.
• Vale was not run locally because the binary is not installed in this environment.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/michael.osuji/dal-834-oauth-scope-docs/account_management/org_settings/mobile_third_party_access