Allow creation and modification of scopes in GN.

tools/gn/functions.cc

Index: tools/gn/functions.cc
diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc
index dcea722ea0dcdf5675b3d39ac6e4f395f00517ca..02129a84e3a141b379d271fc269388280e593e4c 100644
--- a/tools/gn/functions.cc
+++ b/tools/gn/functions.cc
@@ -612,10 +612,12 @@ const char kSetSourcesAssignmentFilter_Help[] =
     "\n"
     "  The sources assignment filter is a list of patterns that remove files\n"
     "  from the list implicitly whenever the \"sources\" variable is\n"
-    "  assigned to. This is intended to be used to globally filter out files\n"
-    "  with platform-specific naming schemes when they don't apply, for\n"
-    "  example, you may want to filter out all \"*_win.cc\" files on non-\n"
-    "  Windows platforms.\n"
+    "  assigned to. This will do nothing for non-lists.\n"
+    "\n"
+    "  This is intended to be used to globally filter out files with\n"
+    "  platform-specific naming schemes when they don't apply, for example\n"
+    "  you may want to filter out all \"*_win.cc\" files on non-Windows\n"
+    "  platforms.\n"
     "\n"
     "  Typically this will be called once in the master build config script\n"
     "  to set up the filter for the current platform. Subsequent calls will\n"