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

sql-statements/sql-statement-show-stats-buckets.md

@@ -21,7 +21,7 @@ Currently, the `SHOW STATS_BUCKETS` statement returns the following 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. |
 
 ## Synopsis
 
@@ -61,4 +61,4 @@ This statement is a TiDB extension to MySQL syntax.
 ## See also
 
 * [`ANALYZE`](/sql-statements/sql-statement-analyze-table.md)
-* [Introduction to Statistics](/statistics.md)
+* [Introduction to Statistics](/statistics.md)

system-variables.md

@@ -1114,7 +1114,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
 ### tidb_analyze_distsql_scan_concurrency <span class="version-mark">New in v7.6.0</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
@@ -1360,7 +1360,7 @@ 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
@@ -4275,7 +4275,7 @@ mysql> desc select count(distinct a) from test.t;
     +-----------------------------------+---------+-----------+-----------------------+---------------------------------+
     ```
 
-- The second example uses `0`, which assumes that 0% of rows will be scanned before the qualified rows are found. 
+- The second example uses `0`, which assumes that 0% of rows will be scanned before the qualified rows are found.
 
     ```sql
     > SET SESSION tidb_opt_ordering_index_selectivity_ratio = 0;