update doc to mark experimental feature

Signed-off-by: Ben Ye <[email protected]>

Author: yeya24

docs/blocks-storage/querier.md

@@ -1441,4 +1441,8 @@ blocks_storage:
     # be out-of-order.
     # CLI flag: -blocks-storage.tsdb.out-of-order-cap-max
     [out_of_order_cap_max: <int> | default = 32]
+
+    # [EXPERIMENTAL] True to enable native histogram.
+    # CLI flag: -blocks-storage.tsdb.enable-native-histograms
+    [enable_native_histograms: <boolean> | default = false]
 ```

docs/blocks-storage/store-gateway.md

@@ -1566,4 +1566,8 @@ blocks_storage:
     # be out-of-order.
     # CLI flag: -blocks-storage.tsdb.out-of-order-cap-max
     [out_of_order_cap_max: <int> | default = 32]
+
+    # [EXPERIMENTAL] True to enable native histogram.
+    # CLI flag: -blocks-storage.tsdb.enable-native-histograms
+    [enable_native_histograms: <boolean> | default = false]
 ```

docs/configuration/config-file-reference.md

@@ -1998,6 +1998,10 @@ tsdb:
   # be out-of-order.
   # CLI flag: -blocks-storage.tsdb.out-of-order-cap-max
   [out_of_order_cap_max: <int> | default = 32]
+
+  # [EXPERIMENTAL] True to enable native histogram.
+  # CLI flag: -blocks-storage.tsdb.enable-native-histograms
+  [enable_native_histograms: <boolean> | default = false]
 ```
 
 ### `compactor_config`