[mlir][ArmSME] Add comments in tile-spills-and-fills.mlir (#91450)

* adds comments in tile-spills-and-fills.mlir
* adds comments in ArmSMEIntrinsicOps.td
* updates test in tile-spills-and-fills.mlir not to return 2D scalable
  vectors (e.g. vector<[4]x[4]xf32>) - that's not supported and not
  needed for that test