docs: add lost _filters param docs

docs/docs/installation/sql-templating.mdx

@@ -30,7 +30,9 @@ made available in the Jinja context:
 For example, to add a time range to a virtual dataset, you can write the following:
 
 ```sql
-SELECT * from tbl where dttm_col > '{{ from_dttm }}' and dttm_col < '{{ to_dttm }}'
+SELECT *
+FROM tbl
+WHERE dttm_col > '{{ from_dttm }}' and dttm_col < '{{ to_dttm }}'
 ```
 
 You can also use [Jinja's logic](https://jinja.palletsprojects.com/en/2.11.x/templates/#tests)
@@ -64,6 +66,41 @@ JINJA_CONTEXT_ADDONS = {
 }
 ```
 
+Default values for jinja templates can be specified via ``Parameters`` menu in the SQL Lab user interface.
+In the UI you can assign a set of parameters as JSON
+
+```json
+{
+  "my_table": "foo"
+}
+```
+The parameters become available in your SQL (example:SELECT * FROM {{ my_table }} ) by using Jinja templating syntax.
+SQL Lab template parameters are stored with the dataset as TEMPLATE PARAMETERS.
+
+There is a special ``_filters`` parameter which can be used to test filters used in the jinja template.
+
+```json
+{
+  "_filters": [
+    {
+      "col": "action_type",
+      "op": "IN",
+      "val": ["sell", "buy"]
+    }
+  ]
+}
+```
+
+```sql
+SELECT action, count(*) as times
+FROM logs
+WHERE action in {{ filter_values('action_type'))|where_in }}
+GROUP BY action
+```
+
+Note ``_filters`` is not stored with the dataset. It's only used within the SQL Lab UI.
+
+
 Besides default Jinja templating, SQL lab also supports self-defined template processor by setting
 the `CUSTOM_TEMPLATE_PROCESSORS` in your superset configuration. The values in this dictionary
 overwrite the default Jinja template processors of the specified database engine. The example below