RFR (XS): 8216366: Add rationale to PER_CPU_SHARES define

[Severin Gehwolf]

Hi,

Could I get reviews for this comment-only change, please. It's meant to
clarify as to why a magic number of 1024 is being used for
PER_CPU_SHARES define for the JVM container support.

Bug: https://bugs.openjdk.java.net/browse/JDK-8216366
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8216366/webrev.01/

More info is in this thread:
http://mail.openjdk.java.net/pipermail/hotspot-dev/2019-January/036087.html

Thanks,
Severin