Use $root:default as a "default" rule

tools/gn/docs/reference.md

Index: tools/gn/docs/reference.md
diff --git a/tools/gn/docs/reference.md b/tools/gn/docs/reference.md
index 9c318b8ecb14fa59f7f45b9a6e9b9b3915eb173f..ebc322dec4eff913a3e126f55b77a3d6426e9741 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -5355,7 +5355,7 @@
 
   root [optional]
       Label of the root build target. The GN build will start by loading the
-      build file containing this target name. This defaults to "//:" which will
+      build file containing this target name. This defaults to label which will
       cause the file //BUILD.gn to be loaded.
 
   secondary_source [optional]
@@ -5861,8 +5861,9 @@
   All generated targets (see "gn help execution") will be added to an implicit
   build rule called "all" so "ninja all" will always compile everything. The
   default rule will be used by Ninja if no specific target is specified (just
-  typing "ninja"). If there is a target named "//:default" it will be the
-  default build rule, otherwise the implicit "all" rule will be used.
+  typing "ninja"). If there is a target named "//:default" (or $root:default in

[brettw, 2017/04/19 19:49:25]

"root" isn't a variable anywhere but the dotfile, so this text is confusing to
me. Maybe just replace this whole sentence with "If there is a target named
"default" in the root build file..."

[Petr Hosek, 2017/04/19 20:20:26]

Done.

+  case the non-default root build target is used) it will be the default build
+  rule, otherwise the implicit "all" rule will be used.
 ```
 
 #### **Phony rules**