Improve explanation in security section.

docs/examples/notebooks/01_notebook_introduction.ipynb

@@ -384,8 +384,8 b''
       "## Security", 
       "", 
       "By default the notebook only listens on localhost, so it does not expose your computer to attacks coming from", 
-      "the internet.  But the basic notebook does not require any authentication, so if you are in a multiuser", 
-      "computer where others can see local ports you should at least protect the notebook with a password.", 
+      "the internet.  By default the notebook does not require any authentication, but you can configure it to", 
+      "ask for a password before allowing access to the files.  ", 
       "", 
       "Furthermore, you can require the notebook to encrypt all communications by using SSL and making all connections", 
       "using the https protocol instead of plain http.  This is a good idea if you decide to run your notebook on",