Fix MLIR FuncOp documentation: declaration must be private (NFC)

Author: joker-eph

mlir/include/mlir/Dialect/Func/IR/FuncOps.td

@@ -246,8 +246,8 @@ def FuncOp : Func_Op<"func", [
 
     ```mlir
     // External function definitions.
-    func.func @abort()
-    func.func @scribble(i32, i64, memref<? x 128 x f32, #layout_map0>) -> f64
+    func.func private @abort()
+    func.func private @scribble(i32, i64, memref<? x 128 x f32, #layout_map0>) -> f64
 
     // A function that returns its argument twice:
     func.func @count(%x: i64) -> (i64, i64)
@@ -256,13 +256,13 @@ def FuncOp : Func_Op<"func", [
     }
 
     // A function with an argument attribute
-    func.func @example_fn_arg(%x: i32 {swift.self = unit})
+    func.func private @example_fn_arg(%x: i32 {swift.self = unit})
 
     // A function with a result attribute
-    func.func @example_fn_result() -> (f64 {dialectName.attrName = 0 : i64})
+    func.func private @example_fn_result() -> (f64 {dialectName.attrName = 0 : i64})
 
     // A function with an attribute
-    func.func @example_fn_attr() attributes {dialectName.attrName = false}
+    func.func private @example_fn_attr() attributes {dialectName.attrName = false}
     ```
   }];