docs: improve grammar and wording in guide/tasks

[dgxo]

🔗 Linked issue

This is not enough of a change to have an issue imo, but I can create one if needed.

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR fixes various grammar issues and rewords certain parts to be more easily understood.
However, this is currently a draft because there are portions that are just incorrect and I'd like some clarification on what to replace them with:

• The (corrected) subtitle of the Tasks page is:

  Nitro tasks allow one-off operations in runtime.

  This doesn't accurately describe tasks. Tasks, as a feature, allow scheduled operations during runtime.
  They can also be programmatically be started, but this is just an extra, not the main intended functionality.

• At the very bottom of the page:

  Nitro tasks can be running multiple times and in parallel.

  This is not true, the paragraph directly above it describes how tasks cannot be ran in parallel.

Please could it be made clear what these bits should actually say, they are both essentially wrong.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.
  • This is the documentation...