Ruby: Update CSRF protection notes in documentation

ruby/ql/src/queries/security/cwe-352/CSRFProtectionDisabled.qhelp

@@ -58,6 +58,11 @@
       for example if parts of the session are memoized. Calling
       <code>protect_from_forgery with: :exception</code> can help to avoid this
       by raising an exception on an invalid CSRF token instead.
+      Note this remains true even in Rails version 5 and later: these versions
+      automatically run <code>protect_from_forgery with: :exception</code>
+      by default, but manually calling <code>protect_from_forgery</code> with
+      no <code>with</code> argument will still downgrade protection to provide an
+      empty session rather than raise an exception.
     </p>
 
   </example>

ruby/ql/src/queries/security/cwe-352/CSRFProtectionNotEnabled.qhelp

@@ -42,6 +42,12 @@
       vulnerability - for example if parts of the session are memoized. Calling
       <code>protect_from_forgery with: :exception</code> can help to avoid this
       by raising an exception on an invalid CSRF token instead.
+
+      Note that Rails versions 5 and later
+      automatically run <code>protect_from_forgery with: :exception</code>
+      by default, but manually calling <code>protect_from_forgery</code> with
+      no <code>with</code> argument will downgrade protection to provide an empty
+      session rather than raise an exception.
     </p>
   </recommendation>