Use $root:default as a "default" rule

tools/gn/docs/reference.md

 # GN Reference
 
 *This page is automatically generated from* `gn help --markdown all`.
 
 ## Contents
 
 *   [Commands](#commands)
     *   [analyze: Analyze which targets are affected by a list of files.](#analyze)
     *   [args: Display or configure arguments declared by the build.](#args)
     *   [check: Check header dependencies.](#check)
@@ skipping 5337 matching lines @@
       If unspecified, the ability to call exec_script is unrestricted.
 
       Example:
         exec_script_whitelist = [
           "//base/BUILD.gn",
           "//build/my_config.gni",
         ]
 
   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]
       Label of an alternate directory tree to find input files. When searching
       for a BUILD.gn file (or the build config file discussed above), the file
       will first be looked for in the source root. If it's not found, the
       secondary source root will be checked (which would contain a parallel
       directory hierarchy).
 
       This behavior is intended to be used when BUILD.gn files can't be checked
@@ skipping 485 matching lines @@
     //tools/gn  ->  //tools/gn:gn
 ```
 ### <a name="ninja_rules"></a>**Ninja build rules**
 
 #### **The "all" and "default" rules**
 
 ```
   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**
 
 ```
   GN generates Ninja "phony" rules for targets in the default toolchain.  The
   phony rules can collide with each other and with the names of generated files
   so are generated with the following priority:
 
     1. Actual files generated by the build always take precedence.
@@ skipping 270 matching lines @@
     *   [--root: Explicitly specify source root.](#--root)
     *   [--runtime-deps-list-file: Save runtime dependencies for targets in file.](#--runtime-deps-list-file)
     *   [--script-executable: Set the executable used to execute scripts.](#--script-executable)
     *   [--threads: Specify number of worker threads.](#--threads)
     *   [--time: Outputs a summary of how long everything took.](#--time)
     *   [--tracelog: Writes a Chrome-compatible trace log to the given file.](#--tracelog)
     *   [-v: Verbose logging.](#-v)
     *   [--version: Prints the GN version number and exits.](#--version)
 ```