Docstring corrections/clarifications for rbenv state

The docstring should be recommending the use of "pkgs" rather than
"names". Also fixed a syntax error in the example, and added a
"versionadded" directive to better clarify the fact that this is a new
state, not available pre-0.16.0.
Merge pull request #5860 from terminalmage/docs

Docstring corrections/clarifications for rbenv state