Clarify the read-once behaviour of System.getenv

The system environment variables are read and cached lazily, on first invocation of getenv. This can lead to some confusion [1], as one could have a reasonable expectation that getenv would read the "live" environment variables on every invocation ( thus seeing any updates or modifications to them ). The specification should document, or even mandate, this behaviour.

[1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-May/047588.html