Add not_needed function

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 735e5f9eadd9afeba60a98dd64826e7044cdb72d..24b78fdee193d1c3845f822b16d929add418d1fb 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -42,6 +42,7 @@
     *   [get_target_outputs: [file list] Get the list of outputs from a target.](#get_target_outputs)
     *   [getenv: Get an environment variable.](#getenv)
     *   [import: Import a file into the current scope.](#import)
+    *   [not_needed: Mark variables from scope as not needed.](#not_needed)
     *   [pool: Defines a pool object.](#pool)
     *   [print: Prints to the console.](#print)
     *   [process_file_template: Do template expansion over a list of files.](#process_file_template)
@@ -598,7 +599,7 @@
 ### <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.
@@ -608,6 +609,9 @@
   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.
 ```
 
@@ -688,9 +692,10 @@
 #### **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.
@@ -2127,6 +2132,25 @@
   # Looks in the current directory.
   import("my_vars.gni")
 ```
+### <a name="not_needed"></a>**not_needed**: Mark variables from scope as not needed.
+
+```
+  not_needed(variable_list_or_star, variable_to_ignore_list = [])
+  not_needed(from_scope, variable_list_or_star,
+             variable_to_ignore_list = [])
+
+  Mark the variables in the current or given scope as not needed, which means
+  you will not get an error about unused variables for these.
+```
+
+#### **Example**
+
+```
+  not_needed("*", [ "config" ])
+  not_needed([ "data_deps", "deps" ])
+  not_needed(invoker, "*", [ "config" ])
+  not_needed(invoker, [ "data_deps", "deps" ])
+```
 ### <a name="pool"></a>**pool**: Defines a pool object.
 
 ```