[MLIR] Add ngraph-opt documentation (#3600)

src/contrib/mlir/tools/ngraph-opt/ngraph_opt.cpp

@@ -13,6 +13,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //*****************************************************************************
+//
+/// \file `ngraph-opt` is a driver for MLIR back-end in nGraph, similar to `opt` and `mlir-opt` for
+/// LLVM and MLIR, respectively. It allows invoking a sequence of arbitrary MLIR passes on a given
+/// input IR. For example, `ngraph-opt my_test.mlir -optA -optC` will run `optA` and `optC`, in that
+/// particular order, on the input IR in file `my_test.mlir` and dump the resulting IR to the
+/// standard output.
+///
+/// `ngraph-opt` is used in LLVM-style LIT tests since it allows invoking a single MLIR pass or a
+/// small sequence of passes without running the whole compiler pipeline. Please, refer to
+/// ngraph_repo_path/tests/mlir/ for examples.
 
 #include "ngraph/check.hpp"