[foreign-memaccess] RFR: 8249879: Split MemorySegment and MemoryAddress (minor javadoc tweak)

[Maurizio Cimadamore]

The javadoc for `MemoryHandles::varHandle` is out of sync with the latest changes, and it does not describe the
coordinates types of the memory access var handle accurately.

Thanks @slowhog for catching this.

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Commit messages:
 - Rectory javadoc of MemoryHandles::varHandle

Changes: https://git.openjdk.java.net/panama-foreign/pull/270/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/270/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249879
  Stats: 11 lines in 1 file changed: 2 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/270.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/270/head:pull/270

PR: https://git.openjdk.java.net/panama-foreign/pull/270