fixup: add documentation justifying the serialization test

Author: abulavin

mlir/test/Conversion/VectorToLLVM/pass-option-serialization.mlir

@@ -1,6 +1,15 @@
-// Simple regression test that ensures ConvertVectorToLLVMPass options remain
-// serialisable. We don't need to actually parse any IR to print the pass
-// options. We just need to provide --dump-pass-pipeline
+// Ensure that ConvertVectorToLLVMPass options remain serialisable.
+
+// This test also allows us to exercise these options (to some extent) even if we
+// don't use them in other Vector to LLVM conversion tests. This is quite relevant
+// for the `Vector` Dialect (and `--convert-vector-to-llvm` pass) as in many cases
+// we use the Transform Dialect (TD) rather than `--convert-vector-to-llvm` for
+// testing. So here we don't check the correctness of the passes, as they're
+// covered by other tests that use TD, but we still provide some test coverage of
+// these pass options.
+
+// We don't need to actually parse any IR to print the pass options. We just need
+// to provide --dump-pass-pipeline
 
 // RUN: mlir-opt --convert-vector-to-llvm --dump-pass-pipeline %s 2>&1 | FileCheck %s --check-prefix=DEFAULT