add docs

lib/compress.js

@@ -1166,6 +1166,8 @@ merge(Compressor.prototype, {
                 && !node.expression.has_side_effects(compressor);
     }
 
+    // may_eq_null()
+    // returns true if this node may evaluate to null or undefined
     (function(def) {
         def(AST_Node, return_true);
         def(AST_Null, return_true);