RFR: 8323159: Consider adding some text re. memory zeroing in Arena::allocate

[Maurizio Cimadamore]

On Mon, 8 Jan 2024 16:16:50 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to add a clarification that an `Arena` always returns zeroed-out segments for `Arena::allocate` methods.
> 
> Note that other overloaded methods refer to the abstract `Arena::allocate` method via implementation notes.

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17308#pullrequestreview-1809670119