[foreign-memaccess+abi] RFR: Add @apiNote to package-info

[Per Minborg]

This PR proposes an additional `@apiNote` saying native memory behaves differently that normal Java objects.

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Commit messages:
 - Add @apiNote to package-info

Changes: https://git.openjdk.org/panama-foreign/pull/751/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=751&range=00
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/751.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/751/head:pull/751

PR: https://git.openjdk.org/panama-foreign/pull/751