RFR: JDK-8284874: Add comment to ProcessHandle/OnExitTest to describe zombie problem [v2]

[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:

  modify comment

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8240/files
  - new: https://git.openjdk.java.net/jdk/pull/8240/files/d893438b..aac07eaf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8240&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8240&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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