Update parallel-execution.md with clarifications on Feature behavior with method-level parallelism

[DrEsteban]

🤔 What's changed?

Updating the sections that call out the considerations that need to be made when using Feature-level behavior and context with method-level parallelism enabled.

• Clarifying that concern mostly exists with method-level parallelism rather than class-level parallelism
• Adding additional details on what the user can expect if they still choose to enable method-level parallelism

⚡️ What's your motivation?

#345

🏷️ What kind of change is this?

• 📖 Documentation (improvements without changing code)

♻️ Anything particular you want feedback on?

• Am I correct in clarifying the concern really only exists for method-level parallelism?
• Are the new clarifications too verbose?

📋 Checklist:

• [ ] I've changed the behaviour of the code
  • [ ] I have added/updated tests to cover my changes.
• [ ] My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.