RFR: 8332003: Clarify javadoc for MemoryLayout::offsetHandle

[Maurizio Cimadamore]

Consider this layout:


MemoryLayout SEQ = MemoryLayout.sequenceLayout(5,
             MemoryLayout.sequenceLayout(10, JAVA_INT));


And the corresponding offset handle:


MethodHandle offsetHandle = SEQ.offsetHandle(PathElement.sequenceLayout(), PathElement.sequenceLayout());


The resulting method handle takes two additional `long` indices. The implementation checks that the dynamically provided indices conform to the bound available at construction: that is, the first index must be < 5, while the second must be < 10. If this is not true, an `IndexOutOfBoundException` is thrown.

However, the javadoc for `MemoryLayout::byteOffsetHandle` doesn't claim anything in this direction. There are only some vague claims in the javadoc for `PathElement::sequenceElement()` and `PathElement::sequenceElement(long, long, long)` which make some claims on which indices are actually allowed, but the text seems more in the tone of a discussion, rather than actual normative text.

I've tweaked the javadoc for `MemoryLayout::byteOffsetHandle` to actually state that the indices will be checked against the "size" of the corresponding open path element (this is a new concept that I also have defined in the javadoc).

I also added a test for `byteOffsetHandle` as I don't think we had a test for that specifically (although this method is tested indirectly, via `MemoryLayout::varHandle`).

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/19158/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19158&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332003
  Stats: 30 lines in 2 files changed: 26 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19158.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19158/head:pull/19158

PR: https://git.openjdk.org/jdk/pull/19158