Correct comment about empty .expected file

Author: owen-mc

shared/util/codeql/util/test/InlineExpectationsTest.qll

@@ -1,7 +1,8 @@
 /**
  * Provides a library for writing QL tests whose success or failure is based on expected results
  * embedded in the test source code as comments, rather than the contents of an `.expected` file
- * (in that the `.expected` file should always be empty).
+ * (in that the `.expected` file should always be empty, except when used via the post-processing
+ * query).
  *
  * To add this framework to a new language, add a new file
  * (usually called `InlineExpectationsTest.qll`) with: