Add notes about jupyter lab build

docs/extensions_dev.md

@@ -82,8 +82,8 @@ jupyter labextension link <path>
 ```
 
 This causes the builder to re-install the source folder before building
-the application files.  You can also link other npm packages that
-you are working on simultaneously; they will be re-installed but not
+the application files.  You can re-build at any time using `jupyter lab build` and it will reinstall these packages.  You can also link other npm packages 
+that you are working on simultaneously; they will be re-installed but not
 considered as extensions if they lack the metadata.
 
 You can see the list of linked extensions using:

docs/extensions_user.md

@@ -63,7 +63,8 @@ any of the JupyterLab commands, or by setting the `JUPYTERLAB_DIR` environment
 variable.  If not specified, it will default to 
 `<sys-prefix/share/jupyter/lab`, where `sys-prefix` is the 
 site-specific directory prefix of the current Python environment.  You can
-query the current application path using `jupyter lab path`.
+query the current application path using `jupyter lab path`.  To create the
+app directory without installing any extensions, run `jupyter lab build`.
 
 The `settings` directory contains `page_config.json` and `build_config.json`
 files.