The accepted answer answered my question at the time, but is now outdated. Back then grunt-cli used grunt, but with another branch. The not-accepted answers
You're correct, the two projects exist because of different versions of grunt.
grunt-cli
is used for grunt version 0.4 (and has an explicit dependency on 0.4), which as of this writing is in alpha development. grunt-0.4 splits the command line interface (grunt-cli) from the API libraries.
The "Getting Started" documentation you found is in the devel branch of the grunt repository, which will be the 0.4 release.
If you're using grunt 0.3.x, then you don't need to install grunt-cli
.