HaikuBook: fix warnings generated by Doxygen 1.18.16.

This version generates some warnings and identified some problems in our
documentation. It is also a bit more vigilant about undocumented elements, so
these are now decorated with a brief description. In the BNotification docs I
used a stray \p to create some whitespace between code and an image. This is
now fixed in the CSS instead.

docs/user/app/Notification.dox

@@ -83,6 +83,8 @@
 
 /*!
 	\class BNotification
+	\ingroup app
+	\ingroup libbe
 	\brief Construct system-default notifications to be displayed to the user.
 
 	Haiku provides a notification system that allows you to display messages to
@@ -121,8 +123,6 @@
 	notification.Send();
 	\endcode
 
-	\p
-
 	\image html notification_intro.png
 
 	Note that in the previous code example, we set a