Clarify SQLALCHEMY_DATABASE_URI in the docs

docs/installation.rst

@@ -98,7 +98,11 @@ of the parameters you can copy / paste in that configuration module: ::
     # Your App secret key
     SECRET_KEY = '\2\1thisismyscretkey\1\2\e\y\y\h'
 
-    # The SQLAlchemy connection string.
+    # The SQLAlchemy connection string to your database backend
+    # This connection defines the path to the database that stores your
+    # caravel metadata (slices, connections, tables, dashboards, ...).
+    # Note that the connection information to connect to the datasources
+    # you want to explore are managed directly in the web UI
     SQLALCHEMY_DATABASE_URI = 'sqlite:////tmp/caravel.db'
 
     # Flask-WTF flag for CSRF