Remove key from session by using session.delete (#31685)

* Remove key from session by using session.delete

You are not deleting a key from session when you assign nil to that key.

* Update guides on how to destroy a user session

In this commit, the user id is removed from session and controller's variables related to the user are nullified.

[Rafael Mendonça França + Rafael Barbolo]

guides/source/action_controller_overview.md

@@ -457,7 +457,9 @@ class LoginsController < ApplicationController
   # "Delete" a login, aka "log the user out"
   def destroy
     # Remove the user id from the session
-    @_current_user = session[:current_user_id] = nil
+    session.delete(:current_user_id)
+    # Clear the memoized current user
+    @_current_user = nil
     redirect_to root_url
   end
 end