grunt test (Run tests)Runs JSHint and runs the QUnit tests headlessly in PhantomJS.
-grunt docs (Run tests and build docs assets)Compiles and tests CSS, JavaScript, and more for running the local documentation via jekyll serve.
grunt docs (Build & test the docs assets)Builds and tests CSS, JavaScript, and other assets which are used when running the documentation locally via jekyll serve.
grunt (Build absolutely everything and run tests)Compiles and minifies CSS and JavaScript, builds the documentation website, runs the HTML5 validator against the docs, regenerates the Customizer assets, and more. Usually only necessary if you're hacking on Bootstrap itself.