Document the Redis Sentinel failover event

[philjlee]

Jira: TXP-277

Follow-up to #3101, which added a redis-sentinel-events dataflow. That PR surfaced two pieces of metadata that contradict the check's actual behavior. This PR corrects both. No code change.

The check does emit events

_process_master_stats calls self.event(...) when a monitored master's IP changes, guarded by if self._masters[master_name] != "" so it does not fire on the first observation of a master:

if self._masters[master_name] != stats['ip']:
    if self._masters[master_name] != "":  # avoid check initialization
        self.increment('redis.sentinel.failover', tags=base_tags)
        self.event({... 'msg_title': '%s failover from %s to %s' ...})

What was wrong

File	Was	Now
README.md	"The Redis's Sentinel check does not include any events."	Describes the failover event and the initialization guard
manifest.json	"creates_events": false	"creates_events": true

The manifest.json half was caught by Codex review on this branch: leaving creates_events: false while the README documents events would keep catalog pages and any manifest-reading tooling advertising that the integration has no events.

creates_events: true matches what other event-emitting integrations in this repo declare (cloudsmith, redisenterprise, cybersixgill_actionable_alerts).

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