docs: improve stream proxy filtering with example

docs/en/latest/admin-api.md

@@ -977,11 +977,13 @@ By default, this API only returns the http plugins. If you need stream plugins,
 
 | Parameter        | Required | Type     | Description | Example  |
 | ---------------- | ------| -------- | ------| -----|
-| remote_addr      | False  | IP/CIDR  | client IP | "127.0.0.1/32" or "127.0.0.1" |
-| server_addr      | False  | IP/CIDR  | server IP | "127.0.0.1/32" or "127.0.0.1"  |
-| server_port      | False  | Integer  | server port | 9090  |
-| sni              | False  | Host     | server name indication | "test.com"  |
 | upstream | False | Upstream | Upstream configuration, see [Upstream](architecture-design/upstream.md) for more details |  |
 | upstream_id | False | Upstream | specify the upstream id, see [Upstream](architecture-design/upstream.md) for more details |             |
+| remote_addr      | False  | IP/CIDR  | Filter option: forward to upstream if client IP matches | "127.0.0.1/32" or "127.0.0.1" |
+| server_addr      | False  | IP/CIDR  | Filter option: forward to upstream if APISIX server IP matches with server_addr | "127.0.0.1/32" or "127.0.0.1"  |
+| server_port      | False  | Integer  | Filter option: forward to upstream if APISIX server port matches with server_port | 9090  |
+| sni              | False  | Host     | server name indication | "test.com"  |
+
+To know more about how the filter works, see the documentation [here](./stream-proxy.md#more-route-match-options)
 
 [Back to TOC](#table-of-contents)

docs/en/latest/stream-proxy.md

@@ -74,7 +74,11 @@ For more use cases, please take a look at [test case](https://github.com/apache/
 
 ## More route match options
 
-And we can add more options to match a route.
+And we can add more options to match a route. Currently stream route configuration supports 3 fields for filtering:
+
+- server_addr: The address of the APISIX server that accepts the L4 stream connection.
+- server_port: The port of the APISIX server that accepts the L4 stream connection.
+- remote_addr: The address of client from which the request has been made.
 
 Here is an example:
 
@@ -92,7 +96,70 @@ curl http://127.0.0.1:9080/apisix/admin/stream_routes/1 -H 'X-API-KEY: edd1c9f03
 }'
 ```
 
-It means APISIX will proxy the request to `127.0.0.1:1995` which the server address is `127.0.0.1` and the server port is equal to `2000`.
+It means APISIX will proxy the request to `127.0.0.1:1995` when the server address is `127.0.0.1` and the server port is equal to `2000`.
+
+Let's take another real world example:
+
+1. Put this config inside `config.yaml`
+
+   ```yaml
+   apisix:
+     stream_proxy: # TCP/UDP proxy
+       tcp: # TCP proxy address list
+         - 9100 # by default uses 0.0.0.0
+         - "127.0.0.10:9101"
+   ```
+
+2. Now run a mysql docker container and expose port 3306 to the host
+
+   ```shell
+   $ docker run --name mysql -e MYSQL_ROOT_PASSWORD=toor -p 3306:3306 -d mysql
+   # check it using a mysql client that it works
+   $ mysql --host=127.0.0.1 --port=3306 -u root -p
+   Enter password:
+   Welcome to the MySQL monitor.  Commands end with ; or \g.
+   Your MySQL connection id is 25
+   ...
+   mysql>
+   ```
+
+3. Now we are going to create a stream route with server filtering:
+
+   ```shell
+   curl http://127.0.0.1:9080/apisix/admin/stream_routes/1 -H 'X-API-KEY: edd1c9f034335f136f87ad84b625c8f1' -X PUT -d '
+   {
+       "server_addr": "127.0.0.10",
+       "server_port": 9101,
+       "upstream": {
+           "nodes": {
+               "127.0.0.1:3306": 1
+           },
+           "type": "roundrobin"
+       }
+   }'
+   ```
+
+   It only forwards the request to the mysql upstream whenever a connection is received at APISIX server `127.0.0.10` and port `9101`. Let's test that behaviour:
+
+4. Making a request to 9100 (stream proxy port enabled inside config.yaml), filter matching fails.
+
+   ```shell
+   $ mysql --host=127.0.0.1 --port=9100 -u root -p
+   Enter password:
+   ERROR 2013 (HY000): Lost connection to MySQL server at 'reading initial communication packet', system error: 2
+
+   ```
+
+   Instead making a request to the APISIX host and port where the filter matching succeeds:
+
+   ```shell
+   mysql --host=127.0.0.10 --port=9101 -u root -p
+   Enter password:
+   Welcome to the MySQL monitor.  Commands end with ; or \g.
+   Your MySQL connection id is 26
+   ...
+   mysql>
+   ```
 
 Read [Admin API's Stream Route section](./admin-api.md#stream-route) for the complete options list.