system variables.md: add a warning message (#6298) (#6312)

system-variables.md

@@ -495,9 +495,13 @@ Constraint checking is always performed in place for pessimistic transactions (d
 
 ### tidb_enable_cascades_planner
 
+> **Warning:**
+>
+> Currently, cascades planner is an experimental feature. It is not recommended that you use it in the production environment.
+
 - Scope: SESSION | GLOBAL
 - Default value: `OFF`
-- This variable is used to control whether to enable the cascades planner, which is currently considered experimental.
+- This variable is used to control whether to enable the cascades planner.
 
 ### tidb_enable_chunk_rpc <span class="version-mark">New in v4.0</span>
 
@@ -534,6 +538,10 @@ Constraint checking is always performed in place for pessimistic transactions (d
 
 ### tidb_enable_fast_analyze
 
+> **Warning:**
+>
+> Currently, `Fast Analyze` is an experimental feature. It is not recommended that you use it in the production environment.
+
 - Scope: SESSION | GLOBAL
 - Default value: `OFF`
 - This variable is used to set whether to enable the statistics `Fast Analyze` feature.