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

[Per Minborg]

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.

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Commit messages:
 - Add in @implSpec that segments are zeroed out

Changes: https://git.openjdk.org/jdk/pull/17308/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17308&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323159
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17308.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17308/head:pull/17308

PR: https://git.openjdk.org/jdk/pull/17308