[DOCS] Incorrect redis caching requirements

[abhi1693]

Description
The docs only mention to install redis package and provides intructions for redis cluster mode. I had to read the source code to understand that I installed the wrong package.

Suggested Fix
Provide additional requirements in the doc: installation of redis-py-cluster

Type of documentation
salt documentation

Location or format of documentation
https://docs.saltproject.io/en/latest/ref/cache/all/salt.cache.redis_cache.html

Additional context
https://github.com/saltstack/salt/blob/master/salt/cache/redis_cache.py#L213

Side note: the error message for redis package is wrong in the source code: https://github.com/saltstack/salt/blob/master/salt/cache/redis_cache.py#L211. This should just say Please install the redis package.

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