Fixes #14628, #14629: Add note to docs about grunt docs command

docs/_includes/getting-started/grunt.html

@@ -24,6 +24,9 @@
   <h3><code>grunt test</code> (Run tests)</h3>
   <p>Runs <a href="http://jshint.com">JSHint</a> and runs the <a href="http://qunitjs.com">QUnit</a> tests headlessly in <a href="http://phantomjs.org">PhantomJS</a>.</p>
 
+  <h3><code>grunt docs</code> (Run tests and build docs assets)</h3>
+  <p>Compiles and tests CSS, JavaScript, and more for running the local documentation via <code>jekyll serve</code>.</p>
+
   <h3><code>grunt</code> (Build absolutely everything and run tests)</h3>
   <p>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.</p>