RFR: 8290036: Define and specify Runtime shutdown sequence [v9]

[Stuart Marks]

> The concept of the shutdown sequence needs to be specified more clearly. This PR adds text for this into the class specification of `java.lang.Runtime`. Also includes adjustments to related areas in `addShutdownHook`, `halt`, and in the `System` and `Thread` classes. The changes here should coordinate with similar changes to JLS 12.8, JVMS 5.7, and the Invocation API chapter of the _JNI Specification._

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  Additional edits to Runtime and Thread specs.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9437/files
  - new: https://git.openjdk.org/jdk/pull/9437/files/6b559c4c..ca369058

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=07-08

  Stats: 24 lines in 2 files changed: 2 ins; 5 del; 17 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