Docs for refresh_events is incorrect

[samuelallan72]

Bug Description

Docs for refresh_events optional kwarg to COSAgentProvider shows that string event names are accepted:

refresh_events=["update-status", "upgrade-charm"],

(https://github.com/canonical/grafana-agent-operator/blob/main/lib/charms/grafana_agent/v0/cos_agent.py#L110)

However this crashes with this error:

RuntimeError: Framework.observe requires a BoundEvent as second parameter, got config-changed

The docs (text, examples, and type hints) should be updated to demonstrate that it takes event classes as values here.

[samuelallan72]

It should also be documented that refresh_events defaults to the charm on.config_changed event - it's not clear because this is set in the init method, not from the default value of the kwarg (None).