Add Documentation for the Too Many Files Open Error

During a recent deployment of an EC2 instance I ran into the following error:

```
...
2026-08-05 09:22:27.9518 ERROR [NewFileWatcherChannel @ filewatcherchannel.go.104] [ssm-agent-worker] [MessageService] [Association] [associationId=] [OutOfProcExecuter] [.2026-08-05T09-22-27.928Z] filewatcher listener encountered error when start watcher: too many open file
...
```

I've seen that quite a few people have this issue. We initially tried to increase `fs.nr_open` and `file-max`, which didn't work. After some further research we eventually stumbled on this comment [^1] from @torson, which resolved our issue:

> It turned out another configuration tweak was needed:
> ```
> echo fs.inotify.max_user_watches=524288 >> /etc/sysctl.conf
> echo fs.inotify.max_user_instances=256 >> /etc/sysctl.conf
> sysctl -p
> ```
> This then ultimately solved the issue.

It would be nice to have this mentioned somewhere, may it be in the FAQ, the troubleshooting guide, or somewhere else easily accessible.

[^1]: https://github.com/aws/amazon-ssm-agent/issues/330#issuecomment-1563229665

Start by locating the repository's existing FAQ or troubleshooting guide, then review issue #330 and its linked comment for the confirmed inotify settings. Document the too-many-open-files symptom, the relevant sysctl configuration, and where users should find it; done means the guidance is accessible in the selected documentation location.

Written by the indexing model from the issue text.