RFR: 8324960: Unsafe.allocateMemory documentation incorrect regarding zero return value

[Brian Burkhalter]

Align the specification of `Unsafe.allocateMemory` with its implementation and with the specification of `Unsafe.reallocateMemory`.

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Commit messages:
 - 8324960: Unsafe.allocateMemory documentation incorrect regarding zero return value

Changes: https://git.openjdk.org/jdk/pull/17643/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17643&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324960
  Stats: 10 lines in 2 files changed: 2 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/17643.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17643/head:pull/17643

PR: https://git.openjdk.org/jdk/pull/17643