Update documentation for PuppetDB 1.2.0
Remove references to Puppet Enterprise which isn't supported

This only removes references regarding installation/upgrade on PE.
Bits about maintaining or interacting with PuppetDB are included, since
that still applies.
Fix dangling colon in docs
Clarify wording about PE and PuppetDB 1.2
Further wordsmith PE support
Merge pull request #482 from nicklewis/update-docs-1.2

Update docs for 1.2