Enhance GN diagnostic tools

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 5c67038b3d7db7d57d72a938038d87c1bb25c831..94e278722c141f8661a307db18fc93c409da5db6 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -778,26 +778,14 @@ const char kVisibility_Help[] =
     "  outside of any target, and the targets will inherit that scope and see\n"
     "  the definition.\n"
     "\n"
-    "Matching:\n"
+    "Patterns\n"
     "\n"
-    "  You can specify \"*\" but the inputs aren't general patterns. The\n"
-    "  following classes of patterns are supported:\n"
+    "  See \"gn help label_pattern\" for more details on what types of\n"
+    "  patterns are supported. If a toolchain is specified, only targets\n"
+    "  in that toolchain will be matched. If a toolchain is not specified on\n"
+    "  a pattern, targets in all toolchains will be matched.\n"
     "\n"
-    "   - Explicit (no wildcard):\n"
-    "       \"//foo/bar:baz\"\n"
-    "       \":baz\"\n"
-    "   - Wildcard target names:\n"
-    "       \"//foo/bar:*\" (any target in the //foo/bar/BUILD.gn file)\n"
-    "       \":*\"  (any target in the current build file)\n"
-    "   - Wildcard directory names (\"*\" is only supported at the end)\n"
-    "       \"*\"  (any target anywhere)\n"
-    "       \"//foo/bar/*\"  (any target in any subdir of //foo/bar)\n"
-    "       \"./*\"  (any target in the current build file or sub dirs)\n"
-    "\n"
-    "  The toolchain (normally an implicit part of a label) is ignored when\n"
-    "  checking visibility.\n"
-    "\n"
-    "Examples:\n"
+    "Examples\n"
     "\n"
     "  Only targets in the current buildfile (\"private\", the default):\n"
     "    visibility = \":*\"\n"