update docs for apex_available_name

Bug: 285138555
Change-Id: Ib987ac7a4995e69cbdbfc77adbf8d2b835e15cc2
diff --git a/apex/apex.go b/apex/apex.go
index 69eea03..5f72441 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -222,7 +222,7 @@
 	ApexType apexPackaging `blueprint:"mutated"`
 
 	// Name that dependencies can specify in their apex_available properties to refer to this module.
-	// If not specified, this defaults to Soong module name.
+	// If not specified, this defaults to Soong module name. This must be the name of a Soong module.
 	Apex_available_name *string
 }