Add definitions to transaction isolation modes

[Rafatcb]

These comments allow VSCode to show a definition for each transaction type when hovering them:

I copied the definition from the official docs, but it doesn't have the read-only definition. I imagine it could be something like:

A transaction sees only data committed before the statement has been executed and has no write permission.

But, as I'm not sure about this definition, I didn't create a comment for it. What do you think?

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Also, I'd like to mention that ISOLATION_READ_COMMITED has a typo, it should be ISOLATION_READ_COMMITTED. Changing it would break the existing code, so I didn't change it in this PR.