feature-guide: update instructions on how to check if a feature is enabled

Author: pietroalbini

feature-guide.md

@@ -114,10 +114,7 @@ In order to implement a new unstable feature, you need to do the following steps
     
     The current version is not actually important – the important version is when you are *stabilizing* a feature.
 4. Prevent usage of the new feature unless the feature gate is set.  
-    You can check it in most places in the compiler using the expression  
-    ```
-    tcx.sess.features().borrow().$feature_name
-    ```
+    You can check it in most places in the compiler using the expression `tcx.features().$feature_name` (or `sess.features_untracked().borrow().$feature_name` if the tcx is unavailable)
     
     If the feature gate is not set, you should either maintain the pre-feature behavior or raise an error, depending on what makes sense.
 5. Add a test to ensure the feature cannot be used without a feature gate, by creating `feature-gate-$feature_name.rs` and `feature-gate-$feature_name.stderr` fiels under the `src/test/ui/feature-gates` directory.