Write more specifically how to add a new record key, etc

## I'm submitting a...

```
[ ] Bug report
[x] Content update
[ ] Process update (build, deployment, ... )
```

## Type of bug / changes

I was trying to send a doc link on how to create a new record key, but it's not a great link - doesn't specifically show you how here: https://github.com/cypress-io/cypress/issues/4948#issuecomment-519405941
