Explain the whoops page

Author: jim-parry

user_guide_src/source/installation/troubleshooting.rst

@@ -52,4 +52,19 @@ requests properly.
 The solution: use Apache to serve your site, or else the built-in
 CodeIgniter equivalent, ``php spark serve`` from your project root.
 
-.. |CodeIgniter4 Welcome| image:: ../images/welcome.png
+.. |CodeIgniter4 Welcome| image:: ../images/welcome.png
+
+What's with an unhelpful "Whoops!" page?
+------------------------------------------------------
+
+You find that your app is displaying a page with "Whoops!" and
+then the text line "We seem to have hit a snag. Please try again later...".
+
+That is a sign that you are in production mode and have hit an
+unrecoverable error, which we don't want to show to the viewer of
+the webapp, for better security.
+
+You can see the error in the debug toolbar display by setting your environment to
+"development" (in `.env`), and reloading the page.
+
+Don't forget to reset the environment to "production" once you fix the problem!