Update gemspec, Modulefile, CHANGELOG for 4.0.2

Without this patch the information displayed on rubygems.org does not
clearly convey the intent of the Gem format of stdlib from the official
puppet module format of stdlib.  This is a problem because end users
might get confused and start installing stdlib from rubygems.org

This patch addresses the problem by making it clear that the gem version
of stdlib is not intended for end users, but rather is specifically
intended to make it easier for other module authors to depend on stdlib
and have that dependency automatically resolved using a Gemfile.

[ci skip]