Fix GUIDE.md regarding StartExternalSegment

[RichVanderwal]

For some reason, the GUIDE.md file tells our customers to use StartExternalSegment as a function that's available on a transaction. We changed this 4.5 years ago.

Rewrite this to make sense for our customers.

Also, it'd be great to know how the GUIDE.md got into this strange state. the StartExternalSegment function didn't even exist when we moved segment starts out of the transaction, and instead provided the transaction as a parameter to the call -- it was just StartSegment at the time.

[RichVanderwal]

@jodeev says: We should thank the customer (point to the explorer hub post) in the PR and if applicable, in release notes.