Improve documentation about reasoning behind the order of operations

Tom Lane <tgl@sss.pgh.pa.us>

Commit: ed5c4e4a14e9f9f4b818521b943bace0d5cd1e01
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2001-07-16T22:43:34Z
Releases: 7.2.1
Improve documentation about reasoning behind the order of operations
in GetSnapshotData, GetNewTransactionId, CommitTransaction, AbortTransaction,
etc.  Correct race condition in transaction status testing in
HeapTupleSatisfiesVacuum --- this wasn't important for old VACUUM with
exclusive lock on its table, but it sure is important now.  All per
pghackers discussion 7/11/01 and 7/12/01.

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