Allow overriding script executable in .gn file

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 580 matching lines @@
 #### **Examples**
 ```
   gn format //some/BUILD.gn
   gn format some\\BUILD.gn
   gn format /abspath/some/BUILD.gn
   gn format --stdin
 ```
 ### <a name="gen:"></a>**gn gen**: Generate ninja files.
 
 ```
-  gn gen [<ide options>] <out_dir>
+  gn gen [--check] [<ide options>] <out_dir>
 
   Generates ninja files from the current tree and puts them in the given output
   directory.
 
   The output directory can be a source-repo-absolute path name such as:
       //out/foo
   Or it can be a directory relative to the current directory such as:
       out/foo
 
+  "gn gen --check" is the same as running "gn check". See "gn help check"
+  for documentation on that mode.
+
   See "gn help switches" for the common command-line switches.
 ```
 
 #### **IDE options**
 
 ```
   GN optionally generates files for IDE. Possibilities for <ide options>
 
   --ide=<ide_name>
       Generate files for an IDE. Currently supported values:
@@ skipping 17 matching lines @@
 #### **Visual Studio Flags**
 
 ```
   --sln=<file_name>
       Override default sln file name ("all"). Solution file is written to the
       root build directory.
 
   --no-deps
       Don't include targets dependencies to the solution. Changes the way how
       --filters option works. Only directly matching targets are included.
+
+  --winsdk=<sdk_version>
+      Use the specified Windows 10 SDK version to generate project files.
+      As an example, "10.0.15063.0" can be specified to use Creators Update SDK
+      instead of the default one.
 ```
 
 #### **Xcode Flags**
 
 ```
   --workspace=<file_name>
       Override defaut workspace file name ("all"). The workspace file is
       written to the root build directory.
 
   --ninja-extra-args=<string>
@@ skipping 23 matching lines @@
   a list of include paths and defines. Because GN does not generate a full
   .cproject definition, it is not possible to properly define includes/defines
   for each file individually. Instead, one set of includes/defines is generated
   for the entire project. This works fairly well but may still result in a few
   indexer issues here and there.
 ```
 
 #### **Generic JSON Output**
 
 ```
-  Dumps target information to JSON file and optionally invokes python script on
-  generated file. See comments at the beginning of json_project_writer.cc and
-  desc_builder.cc for overview of JSON file format.
+  Dumps target information to a JSON file and optionally invokes a
+  python script on the generated file. See the comments at the beginning
+  of json_project_writer.cc and desc_builder.cc for an overview of the JSON
+  file format.
 
   --json-file-name=<json_file_name>
       Overrides default file name (project.json) of generated JSON file.
 
   --json-ide-script=<path_to_python_script>
       Executes python script after the JSON file is generated. Path can be
       project absolute (//), system absolute (/) or relative, in which case the
       output directory will be base. Path to generated JSON file will be first
       argument when invoking script.
 
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       The label of the toolchain. This will match the value of the
       "current_toolchain" variable when inside that target's declaration.
 ```
 
 #### **Examples**
 
 ```
   get_label_info(":foo", "name")
   # Returns string "foo".
 
-  get_label_info("//foo/bar:baz", "gen_dir")
+  get_label_info("//foo/bar:baz", "target_gen_dir")
   # Returns string "//out/Debug/gen/foo/bar".
 ```
 ### <a name="get_path_info"></a>**get_path_info**: Extract parts of a file or directory name.
 
 ```
   get_path_info(input, what)
 
   The first argument is either a string representing a file or directory name,
   or a list of such strings. If the input is a list the return value will be a
   list containing the result of applying the rule to each item in the input.
@@ skipping 3405 matching lines @@
         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
       cause the file //BUILD.gn to be loaded.
 
+  script_executable [optional]
+      Path to specific Python executable or potentially a different language
+      interpreter that is used to execute scripts in action targets and
+      exec_script calls.
+
   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
       in to certain source directories for whatever reason.
 
@@ skipping 776 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)
 ```