Update exportable to handle documentation issues being errors

Follow-up to https://r.android.com/3106478 to have exportable handle
documentation issues being errors in the same way as everything does.

Bug: 343157584
Test: ./gradlew
Change-Id: Iba490e46e2ff202e2ef78f49389319ee8549fe3f

java/droidstubs.go

@@ -1153,6 +1153,9 @@ func (d *Droidstubs) optionalStubCmd(ctx android.ModuleContext, params stubsComm
 		}
 	}
 
+	// Treat documentation issues as warnings, but error when new.
+	cmd.Flag("--error-when-new-category").Flag("Documentation")
+
 	if params.stubConfig.generateStubs {
 		rule.Command().
 			BuiltTool("soong_zip").