reverse_proxy: set client_max_body_size correctly

Without this, it is possible that HTTP uploads will not pass
if the default limit isn’t raised elsewhere.

docs/advanced/reverse_proxy.md

@@ -64,6 +64,9 @@ server {
       proxy_pass http://localhost:5080/;
       proxy_set_header      Host              $host;
       proxy_set_header      X-Forwarded-For   $proxy_add_x_forwarded_for;
+
+      # A bit of headroom over the 16MB accepted by Prosody.
+      client_max_body_size 20MB;
   }
 }
 
@@ -86,6 +89,9 @@ server {
       proxy_ssl_verify      off;
       proxy_set_header      X-Forwarded-Proto https;
       proxy_ssl_server_name on;
+
+      # A bit of headroom over the 16MB accepted by Prosody.
+      client_max_body_size 20MB;
   }
 }
 ```