[chore] Explicitly allow best effort when capturing process creation time

[thompson-tomo]

Progresses #2655

Changes

This adds a note explaining that best effort for creation time is ok.

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Merge requirement checklist

• [ ] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• [ ] Links to the prototypes or existing instrumentations (when adding or changing conventions)