[foreign-memaccess+abi] RFR: Clarify IllegalArgumentException on MemorySegment::[spliterator, elements]

[Maurizio Cimadamore]

In an offline discussion I was made aware that `MemorySegment.spliterator` was not specifying what happened if the size of the layout passed to spliterator() did not match that of the segment.

The newly revised method has a better javadoc, which says a bit more, but stil doesn't specify what happens if the element layout is bigger than the segment.

This small patch rectifies that, and add few tests to check that the implementation issues the expected exceptions.

-------------

Commit messages:
 - Clarify IllegalArgumentException on MemorySegment::[spliterator,elements]

Changes: https://git.openjdk.java.net/panama-foreign/pull/522/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=522&range=00
  Stats: 24 lines in 2 files changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/522.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/522/head:pull/522

PR: https://git.openjdk.java.net/panama-foreign/pull/522