doc: concurrency of gc delete-range (#18359)

system-variables.md

@@ -3007,9 +3007,14 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Type: Integer
 - Default value: `-1`
-- Range: `[1, 256]`
+- Range: `-1` or `[1, 256]`
 - Unit: Threads
-- Specifies the number of threads in the [Resolve Locks](/garbage-collection-overview.md#resolve-locks) step of GC. A value of `-1` means that TiDB will automatically decide the number of garbage collection threads to use.
+- This variable controls the number of concurrent threads during the [Resolve Locks](/garbage-collection-overview.md#resolve-locks) step of the [Garbage Collection (GC)](/garbage-collection-overview.md) process.
+- Starting from v8.3.0, this variable also controls the number of concurrent threads during the [Delete Ranges](/garbage-collection-overview.md#delete-ranges) step of the GC process.
+- By default, this variable is `-1`, allowing TiDB to automatically determine the appropriate number of threads based on workloads.
+- When this variable is set to a number in the range of `[1, 256]`:
+    - Resolve Locks directly uses the value set for this variable as the number of threads.
+    - Delete Range uses one-fourth of the value set for this variable as the number of threads.
 
 ### tidb_gc_enable <span class="version-mark">New in v5.0</span>