Document stubs in cc_library.

Change-Id: I36a9b884a8cc53e6a7192ef159e0cb2fdbb0e511
diff --git a/cc/library.go b/cc/library.go
index 23556b0..7dc925b 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -58,6 +58,7 @@
 
 	Static_ndk_lib *bool
 
+	// Generate stubs to make this library accessible to APEXes.
 	Stubs struct {
 		// Relative path to the symbol map. The symbol map provides the list of
 		// symbols that are exported for stubs variant of this library.