[mlir][emitc][NFC] Add an example to the description of the emitc.literal operation (#128005)

Co-authored-by: Marius Brehler <[email protected]>

Author: EtoAndruwa

mlir/include/mlir/Dialect/EmitC/IR/EmitC.td

@@ -844,6 +844,17 @@ def EmitC_LiteralOp : EmitC_Op<"literal", [Pure]> {
   let description = [{
     The `emitc.literal` operation produces an SSA value equal to some constant
     specified by an attribute.
+
+    Example:
+
+    ```mlir
+    %p0 = emitc.literal "M_PI" : f32
+    %1 = "emitc.add" (%arg0, %p0) : (f32, f32) -> f32
+    ```
+    ```c++
+    // Code emitted for the operation above.
+    float v2 = v1 + M_PI;
+    ```
   }];
 
   let arguments = (ins StrAttr:$value);