Describe the `virtual_host` parameter

Add details about changing the database number while using UNIX 
socket with a Redis broker.

docs/getting-started/brokers/redis.rst

@@ -38,6 +38,11 @@ If a unix socket connection should be used, the URL needs to be in the format::
 
     redis+socket:///path/to/redis.sock
 
+Specifying a different database while using a unix socket is possible by adding the 
+`virtual_host` parameter to the URL::
+
+    redis+socket:///path/to/redis.sock?virtual_host=db_number
+
 .. _redis-visibility_timeout:
 
 Visibility Timeout