Update GN documentation.

tools/gn/docs/language.md

 # GN Language and Operation
 
 [TOC]
 
 ## Introduction
 
 This page describes many of the language details and behaviors.
 
 ### Use the built-in help!
 
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   2. Execute the build config file (this is the default toolchain). In Chrome
      this is `//build/config/BUILDCONFIG.gn`.
   3. Load the `BUILD.gn` file in the root directory.
   4. Recursively load `BUILD.gn` in other directories to resolve all
      current dependencies. If a BUILD file isn't found in the specified
      location, GN will look in the corresponding location inside
      `build/secondary`.
   5. When a target's dependencies are resolved, write out the `.ninja`
      file to disk.
   6. When all targets are resolved, write out the root `build.ninja`
-     file.
+     file. If the top-level build file defines a group called "default"
+     (i.e., `//:default`), GN will tell Ninja to use that for the
+     default build target, rather than building everything. (You can
+     still use `ninja all` to build everything).
 
 ### The build config file
 
 The first file executed is the build config file. The name of this file
 is specified in the `.gn` file that marks the root of the repository. In
 Chrome it is `//build/config/BUILDCONFIG.gn`. There is only one build
 config file.
 
 This file sets up the scope in which all other build files will execute.
 Any arguments, variables, defaults, etc. set up in this file will be
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 GN keeps some GYP concept like "all dependent" settings which work a bit
 differently in Blaze. This is partially to make conversion from the existing
 GYP code easier, and the GYP constructs generally offer more fine-grained
 control (which is either good or bad, depending on the situation).
 
 GN also uses GYP names like "sources" instead of "srcs" since
 abbreviating this seems needlessly obscure, although it uses Blaze's
 "deps" since "dependencies" is so hard to type. Chromium also compiles
 multiple languages in one target so specifying the language type on the
 target name prefix was dropped (e.g. from `cc_library`).