docs: clarify usage of `createContext()`

[Nick-Lucas]

When I first used tRPC I got caught out by a bit of blindness to this part of the docs - it's just a little busy and code-heavy.

Having just seen someone with the same confusion in Discord, here's a crack at improving the clarity

Closes #

🎯 Changes

What changes are made in this PR? Is it a feature or a bug fix?

✅ Checklist

• [ ] I have followed the steps listed in the Contributing guide.
• [ ] If necessary, I have added documentation related to the changes made.
• [ ] I have added or updated the tests related to the changes made.