Update doc

Author: AsTonyshment

docs/advanced/input_files/input-main.md

@@ -1696,7 +1696,7 @@ These variables are used to control the output of properties.
 ### out_freq_td
 
 - **Type**: Integer
-- **Description**: Controls the output interval in **completed electronic evolution steps** during RT-TDDFT calculations. When set to a positive integer $N$, detailed information (see [`out_freq_ion`](#out_freq_ion)) is printed every $N$ electron time-evolution steps (i.e., every $N$ `STEP OF ELECTRON EVOLVE`).
+- **Description**: Controls the output interval in **completed electronic evolution steps** during RT-TDDFT calculations. When set to a positive integer $N$, detailed information (see [`out_freq_ion`](#out_freq_ion)) is printed every $N$ electron time-evolution steps (i.e., every $N$ `STEP OF ELECTRON EVOLVE`). For example, if you wish to output information once per ionic step, you should set `out_freq_td` equal to [`estep_per_md`](#estep_per_md), since one ionic step corresponds to [`estep_per_md`](#estep_per_md) electronic evolution steps.
 - **Default**: 0
 - **Note**: This parameter is **only active in RT-TDDFT mode** (`esolver_type = tddft`). It has no effect in ground-state calculations.