std.ArrayList: add ensureTotalCapacityPrecise and update doc comments

[squeek502]

See #9775 (comment) for more context. If merged, closes #9775.

From the commit messages:

initCapacity did and still does use the ensureTotalCapacityPrecise logic because the initial capacity of an ArrayList is not important in terms of how it grows, so allocating a more exact slice up-front allows for saving memory when the array list never exceeds that initial allocation size. There are use cases where this precise capacity is useful outside of the initCapacity function, though, like in instances where the user does not call the init function themselves but otherwise knows that an ArrayList is empty so calling ensureTotalCapacityPrecise can give the same memory savings that initCapacity would have.

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These [ensureTotalCapacity] calls are all late-initialization of ArrayList's that were initialized outside the current scope. [Replacing them with ensureTotalCapacityPrecise] allows us to still get the potential memory-saving benefits of the 'precision' of initCapacity.