Explain the basis for last sync version selection in comment

This issue tracks the fix to address the code review comment in #226.

Please add info about why this is done to the comment - we want to be able to know why unintuitive stuff like this was done without having to dig up the design doc for everything.

_Originally posted by @Charles-Gagnon in https://github.com/Azure/azure-functions-sql-extension/pull/226#discussion_r931507585_