gh-2032 chore: update TraceID docs to reflect the generation implementation

[ganeshnj]

What and why?

Fixes #2032

How?

Updated docs to match the trace id generation

Review checklist

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration)
• [ ] Make sure each commit and the PR mention the Issue number or JIRA reference
• [ ] Add CHANGELOG entry for user facing changes

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: ganeshnj/docs/trace-id Commit report: 77235c6 Test service: dd-sdk-ios

:white_check_mark: 0 Failed, 1869 Passed, 0 Skipped, 1m 16.2s Total Time :arrow_up: Test Sessions change in coverage: 3 increased, 5 no change