RFR: JDK-8284874: Add comment to ProcessHandle/OnExitTest to describe zombie problem [v3]

[Thomas Stuefe]

> ProcessHandle/OnExitTest is vulnerable to misconfigured systems that do not reap zombies in a timely fashion. [JDK-8284282](https://bugs.openjdk.java.net/browse/JDK-8284282) describes this problem in detail.
> 
> Until we figure out a way to fix that (if at all - see comments in JBS), let us have a clarifying comment in the test itself.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  fix copyright

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8240/files
  - new: https://git.openjdk.java.net/jdk/pull/8240/files/aac07eaf..e8f672c5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8240&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8240&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8240.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8240/head:pull/8240

PR: https://git.openjdk.java.net/jdk/pull/8240