Add documentation to explain how works Redis retry

[Ankhas]

I would like to use Redis retry mechanics on a Redis cluster and standalone instance but I found no documentation about this in the README or documentation site.

I found some arguments to pass to Redis or RedisCluster constructors after searching in redis-py source code, but I don't undestand very well how retry mechanic is working.

I also found similar questions/issues about this here :

• Question: resolving hostname when retry on connection timeout #1833
• timeout connecting to redis hosted on aws ElastiCache #1001

[chayim]

@sav-norem. Wanna?