docs(@angular/cli): update e2e long description

packages/angular/cli/commands/e2e-long.md

@@ -1,2 +1,4 @@
-Must be executed from within a workspace directory.
-When a project name is not supplied, it will execute for all projects.
+The command takes an optional project name, as specified in the `projects` section of the `angular.json` workspace configuration file.
+When a project name is not supplied, executes the `e2e` builder for the default project.
+
+To use the `ng e2e` command, use `ng add` to add a package that implements end-to-end testing capabilities. Adding the package automatically updates your workspace configuration, adding an `e2e` [CLI builder](guide/cli-builder).