RFR: 8290036: Define and specify Runtime shutdown sequence [v2]

[Stuart Marks]

> Initial edits to addShutdownHook from Alex.
> 
> See [JDK-8290036](https://bugs.openjdk.org/browse/JDK-8290036).

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

  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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9437/files
  - new: https://git.openjdk.org/jdk/pull/9437/files/4d7d6f73..b23a7321

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9437&range=00-01

  Stats: 53 lines in 2 files changed: 6 ins; 9 del; 38 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