gn: Mention complete_static_lib in `gn help static_library`.

BUG=none

Review-Url: https://codereview.chromium.org/1975393003
Cr-Original-Commit-Position: refs/heads/master@{#393681}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4f380b7cbb12e5ce75960857e9f8490f86cbdf9d
diff --git a/tools/gn/functions_target.cc b/tools/gn/functions_target.cc
index cc863ac..a5f5574 100644
--- a/tools/gn/functions_target.cc
+++ b/tools/gn/functions_target.cc
@@ -656,6 +656,7 @@
     "\n"
     "Variables\n"
     "\n"
+    "complete_static_lib\n"
     CONFIG_VALUES_VARS_HELP
     DEPS_VARS
     DEPENDENT_CONFIG_VARS