Clarify difference between ABORT_REASON_ABORT_SPAN and ABORT_REASON_ABORTED_RECORD_FOUND

[rmloveland]

Richard Loveland (rmloveland) commented:

Consider updating the Transaction retry error reference page with the following information provided by @andreimatei:

They both indicate that the txn encountering that error was aborted by another txn.
One of the two error reasons is returned based on what operation discovers the fact that the txn was aborted:

• ABORT_REASON_ABORT_SPAN is returned by a read going to the same range as a previous write in that txn
• ABORT_REASON_ABORTED_RECORD_FOUND is returned by the attempt to commit the txn, and by the periodic heartbeating of the txn record done in the background by the gateway node.

Jira Issue: DOC-1195

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