Clarify comment for activation test

Author: koesie10

extensions/ql-vscode/src/vscode-tests/no-workspace/activation/activation.test.ts

@@ -1,5 +1,7 @@
-// This file needs to be located in a separate directory. This will ensure that this
-// test is run at the start-up of a new VSCode instance.
+// This file needs to be located in a separate directory from all other tests. The jest-runner-vscode will
+// create a new VSCode instance for every directory containing tests, so this will ensure that this
+// test is run at the start-up of a new VSCode instance. No other files should be located in this directory since
+// those may activate the extension before this test is run.
 
 import * as vscode from "vscode";