Incorrect description for `az redis firewall-rules list`

# background
I try to export all the firewall rules via azure cli

# command
`az redis firewall-rules list -g my-resource -n my-redis-name`

# expected
It will list out all the firewall rules based on the `-g` and `-n` across diff subscription

# actual
It only searches the resources from the default subscription with `-g` and `-n` criteria

# resolution
change this line
```
--subscription
Name or ID of subscription. You can configure the default subscription using az account set -s NAME_OR_ID.
Please specify it, otherwise, only the default subscription will be searched.
```

# reason
Unlike the `az mysql server firewall-rule list [--ids]` we can offer the ids to filter the targeted resources. It does not share the same behaviors. An engineer like me cannot filter a resource from the `resource id` directly.

It is better to support `az redis firewall-rules list --ids my-resource-id0,my-resource-id1`

## references
1. https://docs.microsoft.com/en-us/cli/azure/mysql/server/firewall-rule?view=azure-cli-latest#az_mysql_server_firewall_rule_list

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: a46d79d4-8d2a-5e62-0e5b-33b6f0b04823
* Version Independent ID: eae0fda6-6565-a785-8a8e-74626711d525
* Content: [az redis firewall-rules](https://docs.microsoft.com/en-us/cli/azure/redis/firewall-rules?view=azure-cli-latest#az_redis_firewall_rules_list)
* Content Source: [latest/docs-ref-autogen/redis/firewall-rules.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/redis/firewall-rules.yml)
* Service: **cache**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**