RFR: 8290036: adjustments to specification of Runtime::addShutdownHook

[Stuart Marks]

Initial edits to addShutdownHook from Alex.

See [JDK-8290036](https://bugs.openjdk.org/browse/JDK-8290036).

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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