HaikuBook: Add documentation for Password and Key Storage API

The current implementation of the keystore_server is not perfect. While the
source has many seeds for a future of having keyrings encrypted, and having
more fine grained permissions, it is far from complete.

The main arguments for adding documentation about this new but incomplete
functionality is that while it is incomplete, the API is part of the public
headers, and there are some legitimate use cases for developers.

The documentation aims to give the proper amount of caution to any developer
that is considering using this API.

Change-Id: I154a3f8374b22dc6929758cba7ba810833bcfe9d

docs/user/book.dox

@@ -102,9 +102,16 @@
 		- BApplication
 		- BClipboard
 		- BCursor
+		- BLaunchRoster
+		- BNotification
 		- BPropertyInfo
 		- BRoster
 
+	A third special category is the \link app_keystore Password and Key storage
+	API:\endlink
+		- BKey
+		- BKeyStore
+
 
 	\defgroup game Game Kit
 	\brief The Game Kit provides classes for producing game sounds and