Improve the documentation in the android/sdk.go file

Makes it adhere to the go standard practice of prefixing documentation
comments with the name of the type/func/method.

Bug: 195754365
Test: m nothing
Change-Id: Idf3fe827edc9b6d67d12a99a4b27539ac938ea95

sdk/update.go

@@ -765,6 +765,8 @@ type propertyTag struct {
 	name string
 }
 
+var _ android.BpPropertyTag = propertyTag{}
+
 // A BpPropertyTag to add to a property that contains references to other sdk members.
 //
 // This will cause the references to be rewritten to a versioned reference in the version