8381012: Add warning for == for MemorySegment.NULL

[minborg]

This PR proposes to improve the documentation for MemorySegment.NULL, adding an @apiNote discouraging the use of == on MemorySegment.NULL.

This PR superseeds #30501

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Progress

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Issue

• JDK-8381012: Add warning for == for MemorySegment.NULL (Enhancement - P4)

Reviewers

• Chen Liang (@liach - Reviewer)

Reviewing

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[openjdk]

@minborg This change now passes all automated pre-integration checks.

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8381012: Add warning for == for MemorySegment.NULL

Reviewed-by: liach

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At the time when this comment was updated there had been 7 new commits pushed to the master branch:

• e6bce14: 8381765: AArch32: Capstone hsdis does not build
• d68065e: 8365498: jdk/jfr/event/os/TestCPULoad.java fails with Expected at least one event
• 9a1f7d4: 8326205: Grouping frequently called C2 nmethods in CodeCache
• ... and 4 more: https://git.openjdk.org/jdk/compare/289f3e2f6236e7b80c16443d5c3547b3c28010fe...master

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[mlbridge]

Webrevs

• 00: Full (5527d245)

[ryanhallock]

Is this api note not already covered if the implementations are meant to be treated as value based, as in no identity comparisons currently?

Though I agree its currently an easy logic error currently.

[liach]

Looks reasonable. Since this is api notes, don't think we need a CSR.