Initial edits to addShutdownHook from Alex. See [JDK-8290036](https://bugs.openjdk.org/browse/JDK-8290036).
------------- Commit messages: - Additional wording changes to Runtime specs. - HTML fixups; updates in response to review comments. - Updates to Runtime class spec, exit, halt, and System.exit - Merge branch 'master' into JDK-8290036-Runtime-addShutdownHook-spec - First cut at new shutdown sequence text. - Initial edits to addShutdownHook from Alex Changes: https://git.openjdk.org/jdk/pull/9437/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8290036 Stats: 140 lines in 2 files changed: 56 ins; 45 del; 39 mod Patch: https://git.openjdk.org/jdk/pull/9437.diff Fetch: git fetch https://git.openjdk.org/jdk pull/9437/head:pull/9437 PR: https://git.openjdk.org/jdk/pull/9437