Add "outputs" to generated_file documentation.

This variable is required but was previously not listed in the list of
variables used by the target type.

Change-Id: Ib8cd9ff367b692f118a6c2a7a2fe360d150e0166
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17640
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Brett Wilson <brettw@google.com>

docs/reference.md

diff --git a/docs/reference.md b/docs/reference.md
index c482630..82cfd80 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -1954,7 +1954,8 @@
   General: check_includes, configs, data, friend, inputs, metadata,
            output_extension, output_name, public, sources, testonly,
            visibility
-  Generated file: contents, data_keys, rebase, walk_keys, output_conversion
+  Generated file: contents, data_keys, rebase, walk_keys, output_conversion,
+                  outputs
 ```
 
 #### **Example (metadata collection)**

src/gn/functions_target.cc

diff --git a/src/gn/functions_target.cc b/src/gn/functions_target.cc
index 7e262ee..df43109 100644
--- a/src/gn/functions_target.cc
+++ b/src/gn/functions_target.cc
@@ -971,7 +971,8 @@
 
 )" DEPENDENT_CONFIG_VARS DEPS_VARS GENERAL_TARGET_VARS
 
-    R"(  Generated file: contents, data_keys, rebase, walk_keys, output_conversion
+    R"(  Generated file: contents, data_keys, rebase, walk_keys, output_conversion,
+                  outputs
 
 Example (metadata collection)