The minimum theoretical value of prune target is zero, but in practice, it is 510, so small rescans and reorgs are still possible. Clients will not let you set below the minimum of 550 MB. A help message explaining -prune and other command-line options comes with Bitcoin Core. Pruning deletes old transaction data that moves coins around and stores only the origin and current location of unspent transactions. There is a "natural" amount of megabytes for a fully pruned blockchain, which should be defined by the final amount of unspent coins. However, the number of megabytes to allot for raw block and undo data can be configured. The release notes were edited to explain why N must be given and what a safe value of N is, as well as mention whether there is an auto setting for N to ensure it chooses a safe value on its own.