Differentiate between build and serve's intended destinations

docs/_docs/usage.md

@@ -23,8 +23,8 @@ Here are some of the most common commands:
 
 * `jekyll new PATH` - Creates a new Jekyll site with default gem-based theme at specified path. The directories will be created as necessary.
 * `jekyll new PATH --blank` - Creates a new blank Jekyll site scaffold at specified path.
-* `jekyll build` or `jekyll b` - Performs a one off build your site to `./_site` (by default).
-* `jekyll serve` or `jekyll s` - Builds your site any time a source file changes and serves it locally.
+* `jekyll build` or `jekyll b` - Performs a one off build your site to `./_site`, using the production configuration (by default).
+* `jekyll serve` or `jekyll s` - Builds your site any time a source file changes and serves it locally, using placeholders for the site URLs to enable it to be served locally (by default). Should not be used to generate a site copy that will be deployed to a live website.
 * `jekyll clean` - Removes all generated files: destination folder, metadata file, Sass and Jekyll caches.
 * `jekyll help` - Shows help, optionally for a given subcommand, e.g. `jekyll help build`.
 * `jekyll new-theme` - Creates a new Jekyll theme scaffold.