Update Service Ping documentation to include note about redis tracking

This issue is a follow up to !67103 (comment 644945482)

Goal

In spirit of transparency we should include note within Service Ping Redis and Redis Hll counters documentation that User.single_user&.requires_usage_stats_consent? is not checked during data tracking stage due to performance reasons, which might result in some key that corresponds to those counters would be present in Redis even if usage_stats_consent is still required. However we should also highlight that no metric would be collected from Redis and reported back to GitLab as long as usage_stats_consent is required.