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

Summary

Specify that segments allocated from an arena created using a factory in the Arena class are always zeroed out.

Problem

Previously, there was no specification of the contents of segments allocated from an arena created using a factory in the Arena class. Such factories are:

• Arena::ofConfined()
• Arena::ofShared()
• Arena::ofAuto()

Solution

Specify that segments allocated from an arena created using a factory in the Arena class are always zeroed out. This is also the behavior that is currently implemented (so, no change in the API implementation is required).

Specification

diff --git a/src/java.base/share/classes/java/lang/foreign/Arena.java b/src/java.base/share/classes/java/lang/foreign/Arena.java
index 195f8a44db6e..280338fbcbdd 100644
--- a/src/java.base/share/classes/java/lang/foreign/Arena.java
+++ b/src/java.base/share/classes/java/lang/foreign/Arena.java
@@ -219,6 +219,9 @@ public interface Arena extends SegmentAllocator, AutoCloseable {
      * Segments allocated with the returned arena can be
      * {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
      * Calling {@link #close()} on the returned arena will result in an {@link UnsupportedOperationException}.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      *
      * @return a new arena that is managed, automatically, by the garbage collector
      */
@@ -231,6 +234,9 @@ static Arena ofAuto() {
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
      *          Calling {@link #close()} on the returned arena will result in
      *          an {@link UnsupportedOperationException}.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena global() {
         class Holder {
@@ -243,6 +249,9 @@ class Holder {
      * {@return a new confined arena} Segments allocated with the confined arena can be
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by the thread
      *          that created the arena, the arena's <em>owner thread</em>.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena ofConfined() {
         return MemorySessionImpl.createConfined(Thread.currentThread()).asArena();
@@ -251,6 +260,9 @@ static Arena ofConfined() {
     /**
      * {@return a new shared arena} Segments allocated with the global arena can be
      *          {@linkplain MemorySegment#isAccessibleBy(Thread) accessed} by any thread.
+     * <p>
+     * Memory segments {@linkplain #allocate(long, long) allocated} by the returned arena
+     * are zero-initialized.
      */
     static Arena ofShared() {
         return MemorySessionImpl.createShared().asArena();```