fix: update

Author: 0xPoe

system-variables.md

@@ -1130,6 +1130,11 @@ 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>
 
+> **Note:**
+>
+> Before v7.6.0, it controlled by the variable `tidb_distsql_scan_concurrency`.
+> So please consider change the value of `tidb_distsql_scan_concurrency` if you want to adjust the concurrency of scanning TiKV regions in versions earlier than v7.6.0.
+
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
@@ -1144,7 +1149,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 - Scope: SESSION | GLOBAL
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
-- Default value: `2`.
+- Default value: `2`. The default value is `1` for v7.4.0 and earlier versions.
 - Range: `[1, 128]`. Before v8.4.0, the value range is `[1, 18446744073709551615]`.
 - For manual/auto analyze: used to control the concurrency for saving analyze results (writing TopN and histograms to system tables).