[foreign-memaccess+abi] RFR: Clarify javadoc w.r.t. exceptions thrown by a memory access var handle (part two)

[Maurizio Cimadamore]

This is a follow-up on https://git.openjdk.org/panama-foreign/pull/869 which fixes two issues:
* the rectified javadoc still did not mention possibility of `WrongThreadException`
* `MemoryLayout::sliceHandle` needs similar clarifications (albeit some exceptions can't occur there)

To make the javadoc a bit more streamlined, I've collected all the checks in bullet lists.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/872/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=872&range=00
  Stats: 25 lines in 1 file changed: 12 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/872.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/872/head:pull/872

PR: https://git.openjdk.org/panama-foreign/pull/872