Clarify a bit more what apex_available means.

Test: n/a - comment change only
Change-Id: I5311dca32a4e43c3765a3ea26d2b8781ce113433
diff --git a/android/apex.go b/android/apex.go
index 8482dc2..6c0d47a 100644
--- a/android/apex.go
+++ b/android/apex.go
@@ -82,7 +82,10 @@
 }
 
 type ApexProperties struct {
-	// Availability of this module in APEXes. Only the listed APEXes can include this module.
+	// Availability of this module in APEXes. Only the listed APEXes can contain
+	// this module. If the module has stubs then other APEXes and the platform may
+	// access it through them (subject to visibility).
+	//
 	// "//apex_available:anyapex" is a pseudo APEX name that matches to any APEX.
 	// "//apex_available:platform" refers to non-APEX partitions like "system.img".
 	// Default is ["//apex_available:platform", "//apex_available:anyapex"].