(DOC-565) Improve description of package type's package_settings property

This commit aims all scrutiny at the provider documentation, and clarifies our
advice for provider developers.
(DOC-698) Revise doc text for modulepath, environmentpath, and friends

These settings wanted some more clarity about the nature of the deprecation and
the nature of the recommended path forward. I also took the chance to move away
from links.puppetlabs.com for a URL whose permanent address we know today.
(maint) Update dir env links in deprecation warning and example env README file

Since we know the permanent URL of this page, we should use it directly.
Merge pull request #2540 from nfagerlund/apr_2014_docs_fixes

Apr 2014 docs fixes