RFR: 8287748: Fix issues in java.lang.foreign package javadoc

[Maurizio Cimadamore]

This patch fixes a couple of issues in the package-level javadoc of the foreign API.
I've also dropped uses of `var`, which I think are confusing so early in the game.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/jdk/pull/9005/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9005&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287748
  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9005.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9005/head:pull/9005

PR: https://git.openjdk.java.net/jdk/pull/9005