doc: Deprecate require.extensions

doc/api/globals.markdown

@@ -65,12 +65,24 @@ value from this object, the next `require` will reload the module.
 
 * {Array}
 
+    Stability: 0 - Deprecated
+
 Instruct `require` on how to handle certain file extensions.
 
 Process files with the extension `.sjs` as `.js`:
 
     require.extensions['.sjs'] = require.extensions['.js'];
 
+**Deprecated**  In the past, this list has been used to load
+non-JavaScript modules into Node by compiling them on-demand.
+However, in practice, there are much better ways to do this, such as
+loading modules via some other Node program, or compiling them to
+JavaScript ahead of time.
+
+Since the Module system is locked, this feature will probably never go
+away.  However, it may have subtle bugs and complexities that are best
+left untouched.
+
 ## __filename
 
 <!-- type=var -->