RFR: 8290036: Define and specify Runtime shutdown sequence [v8]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 13 additional commits since the last revision:

 - Merge branch 'master' into JDK-8290036-Runtime-addShutdownHook-spec
 - Minor adjustments
 - Revise Implementation Note discussing JNI Invocation API.
 - Updates from Alan, David, and Alex.
 - More edits from Alex's suggestions.
 - More updates after discussion with Alex.
 - Updates after conversation with Alan:
   - specify that starting a shutdown hook explicitly has an unspecified
     effect on the shutdown sequence
   - link Thread class doc to shutdown sequence
   - link to Thread.UncaughtExceptionHandler
   - clarify that only live non-daemon threads are significant
   - use "thread termination" to conform to the text in the Thread class
   - adjust line breaks and whitespace
 - Additional wording changes to Runtime specs.
 - HTML fixups; updates in response to review comments.
 - Updates to Runtime class spec, exit, halt, and System.exit
 - ... and 3 more: https://git.openjdk.org/jdk/compare/4b81849d...6b559c4c

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9437/files
  - new: https://git.openjdk.org/jdk/pull/9437/files/9796557d..6b559c4c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=06-07

  Stats: 190478 lines in 3326 files changed: 95187 ins; 75075 del; 20216 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