Update GN documentation.

tools/gn/function_foreach.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/err.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 
 namespace functions {
 
 const char kForEach[] = "foreach";
 const char kForEach_HelpShort[] =
     "foreach: Iterate over a list.";
 const char kForEach_Help[] =
     "foreach: Iterate over a list.\n"
     "\n"
     "  foreach(<loop_var>, <list>) {\n"
     "    <loop contents>\n"
     "  }\n"
     "\n"
     "  Executes the loop contents block over each item in the list,\n"
-    "  assigning the loop_var to each item in sequence.\n"
+    "  assigning the loop_var to each item in sequence. The loop_var will be\n"
+    "  a copy so assigning to it will not mutate the list.\n"
     "\n"
     "  The block does not introduce a new scope, so that variable assignments\n"
     "  inside the loop will be visible once the loop terminates.\n"
     "\n"
     "  The loop variable will temporarily shadow any existing variables with\n"
     "  the same name for the duration of the loop. After the loop terminates\n"
     "  the loop variable will no longer be in scope, and the previous value\n"
     "  (if any) will be restored.\n"
     "\n"
     "Example\n"
@@ skipping 77 matching lines @@
     scope->SetValue(loop_var, old_loop_value, old_loop_value.origin());
   } else {
     // Loop variable was undefined before loop, delete it.
     scope->RemoveIdentifier(loop_var);
   }
 
   return Value();
 }
 
 }  // namespace functions