docs: update mqtt-proxy doc to enable stream mode (#12712)

Author: Baoyuantop

docs/en/latest/plugins/mqtt-proxy.md

@@ -49,6 +49,7 @@ To enable the Plugin, you need to first enable the `stream_proxy` configuration
     router:
         http: 'radixtree_uri'
         ssl: 'radixtree_sni'
+    proxy_mode: http&stream
     stream_proxy:                 # TCP/UDP proxy
       tcp:                        # TCP proxy port list
         - 9100

docs/zh/latest/plugins/mqtt-proxy.md

@@ -49,6 +49,7 @@ description: 本文档介绍了 Apache APISIX mqtt-proxy 插件的信息，通
     router:
         http: 'radixtree_uri'
         ssl: 'radixtree_sni'
+    proxy_mode: http&stream
     stream_proxy:                 # TCP/UDP proxy
       tcp:                        # TCP proxy port list
         - 9100