Improve comment describing the |reportLeftRecursion| pass

lib/compiler/passes/report-left-recursion.js

@@ -3,7 +3,18 @@ var arrays       = require("../../utils/arrays"),
     asts         = require("../asts"),
     visitor      = require("../visitor");
 
-/* Checks that no left recursion is present. */
+/*
+ * Reports left recursion in the grammar, which prevents infinite recursion in
+ * the generated parser.
+ *
+ * Both direct and indirect recursion is detected. The pass also correctly
+ * reports cases like this:
+ *
+ *   start = "a"? start
+ *
+ * In general, if a rule reference can be reached without consuming any input,
+ * it can lead to left recursion.
+ */
 function reportLeftRecursion(ast) {
   function matchesEmptyTrue()  { return true;  }
   function matchesEmptyFalse() { return false; }