add GatherD comments

mindspore/ops/operations/array_ops.py

@@ -4649,6 +4649,17 @@ class GatherD(PrimitiveWithInfer):
     """
     Gathers values along an axis specified by dim.
 
+    For a 3-D tensor, the output is:
+        output[i][j][k] = x[index[i][j][k]][j][k]  # if dim == 0
+
+        output[i][j][k] = x[i][index[i][j][k]][k]  # if dim == 1
+
+        output[i][j][k] = x[i][j][index[i][j][k]]  # if dim == 2
+
+    If `x` is an n-D tensor with shape :math:`(z_0, z_1, ..., z_i, ..., z_{n-1})` and `dim` = i,
+    the `index` must be an n-D tensor with shape :math:`(z_0, z_1, ..., y, ..., z_{n-1})`
+    where `y`>=1 and the output will have the same shape as `index`.
+
     Inputs:
         - **x** (Tensor) - The source tensor.
         - **dim** (int) - The axis along which to index. It must be int32. Only constant value is allowed.