statistics: mark NDV as deprecated and update variable scopes (#18499) (

#18524)

statistics.md

@@ -654,7 +654,7 @@ Currently, the `SHOW STATS_BUCKETS` statement returns the following 11 columns:
 | `repeats` | The occurrence number of the maximum value |
 | `lower_bound` | The minimum value |
 | `upper_bound` | The maximum value |
-| `ndv` | The number of different values in the bucket. When `tidb_analyze_version` = `1`, `ndv` is always `0`, which has no actual meaning. |
+| `Ndv` | The number of distinct values in the bucket. This field is deprecated and always shows `0` due to its inaccurate value. |
 
 ### Top-N information
 

system-variables.md

@@ -1323,7 +1323,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 
 ### tidb_build_sampling_stats_concurrency <span class="version-mark">New in v7.5.0</span>
 
-- Scope: GLOBAL
+- Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Type: Integer