Reference configs variable from config help in GN

This wasn't listed as a valid flag in the "gn help config" output.

This also updates the style of the help message for "config" (newlines after headings, no colons).

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1510663002

Cr-Original-Commit-Position: refs/heads/master@{#363643}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 90cdb30cd5ddb90618f4dae90579bc8140ea9abb

tools/gn/functions.cc

diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc
index 18fd0b3..5101a41 100644
--- a/tools/gn/functions.cc
+++ b/tools/gn/functions.cc
@@ -261,13 +261,17 @@
     "   4. All dependent configs from a breadth-first traversal of the\n"
     "      dependency tree in the order that the targets appear in \"deps\".\n"
     "\n"
-    "Variables valid in a config definition:\n"
-    CONFIG_VALUES_VARS_HELP
+    "Variables valid in a config definition\n"
     "\n"
-    "Variables on a target used to apply configs:\n"
+    CONFIG_VALUES_VARS_HELP
+    "  Nested configs: configs\n"
+    "\n"
+    "Variables on a target used to apply configs\n"
+    "\n"
     "  all_dependent_configs, configs, public_configs\n"
     "\n"
-    "Example:\n"
+    "Example\n"
+    "\n"
     "  config(\"myconfig\") {\n"
     "    includes = [ \"include/common\" ]\n"
     "    defines = [ \"ENABLE_DOOM_MELON\" ]\n"