[wdoc] Fix obsolete documentation (related to #3694152)

It used to refer to the secret key defined in all-in-one.conf as:
	[MAIN]
	secret-key="my secret key"
Unless I am mistaken, you should now point to an external file
that contains the secret key.
	[TRUSTEDAUTH]
	trustedauth-secret-key-file="/etc/application/secret-key"

wdoc/setup.rst

@@ -139,18 +139,19 @@ your application ``all-in-one.conf`` file.
 Modify your ``all-in-one.conf``
 -------------------------------
 
-Add the ``secret-key`` option in the ``main`` option group. The value
-should be a secret key that is used to secure connection between the
-web part of the CubicWeb application and the data part of the
-application (they may live on different machines, communicating via
-pyro).
+Add the ``trustedauth-secret-key-file`` option in the ``trustedauth``
+option group. The value should point to a file containing the secret key
+that is used to secure the connection between the web part of the CubicWeb
+application and the data part of the application (they may live on
+different machines, communicating via pyro). Do not forget to create the
+file. The secret key should be between 1 and 32 characters long.
 
 .. sourcecode:: ini
 
    # ...
 
-   [MAIN]
-   secret-key="my secret key"
+   [TRUSTEDAUTH]
+   trustedauth-secret-key-file="/etc/application/secret-key"
 
    # ...