Update c_api LGBM_SampleIndices() comment. (#4490)

* Update c_api LGBM_SampleIndices() comment.

rand.Sample() now returns exactly given number of samples, thus the
comment should be fixed.

* Update include/LightGBM/c_api.h
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

include/LightGBM/c_api.h

@@ -71,7 +71,7 @@ LIGHTGBM_C_EXPORT int LGBM_GetSampleCount(int32_t num_total_row,
  * \param num_total_row Number of total rows
  * \param parameters Additional parameters, namely, ``bin_construct_sample_cnt`` and ``data_random_seed`` are used to produce the output
  * \param[out] out Created indices, type is int32_t
- * \param[out] out_len Number of indices. This may be less than the one returned by ``LGBM_GetSampleCount``
+ * \param[out] out_len Number of indices
  * \return 0 when succeed, -1 when failure happens
  */
 LIGHTGBM_C_EXPORT int LGBM_SampleIndices(int32_t num_total_row,

python-package/lightgbm/basic.py

@@ -1223,7 +1223,8 @@ class Dataset:
             ptr_data,
             ctypes.byref(actual_sample_cnt),
         ))
-        return indices[:actual_sample_cnt.value]
+        assert sample_cnt == actual_sample_cnt.value
+        return indices
 
     def _init_from_ref_dataset(self, total_nrow: int, ref_dataset: 'Dataset') -> 'Dataset':
         """Create dataset from a reference dataset.