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

[Maurizio Cimadamore]

We do not state very clearly which exceptions can occur when calling a memory segment var handle.
This patch rectifies that.

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Commit messages:
 - Clarify javadoc w.r.t. exceptions thrown by a memory access var handle

Changes: https://git.openjdk.org/panama-foreign/pull/869/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=869&range=00
  Stats: 14 lines in 1 file changed: 9 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/869.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/869/head:pull/869

PR: https://git.openjdk.org/panama-foreign/pull/869