docs(ruby): Document attempt_threshold for sentry-sidekiq (#19329)

## DESCRIBE YOUR PR

`attempt_threshold` is only mentioned in the changelog, so there's no
reference for it in the docs. Someone asked about it on the PR that
added it, and the maintainer said it should be documented:

> good point, we should definitely document this and related features as
it's kinda tricky 😅 I'll make sure we get it done!
>
> — [getsentry/sentry-ruby#2503
(comment)](getsentry/sentry-ruby#2503 (comment))

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 2 people

docs/platforms/ruby/common/configuration/integration_options.mdx

@@ -75,3 +75,12 @@ config.delayed_job.report_after_job_retries = true
 ```
 config.sidekiq.report_after_job_retries = true
 ```
+
+`attempt_threshold`
+
+`sentry-sidekiq` skips error reporting until a job reaches the given attempt. Set it in the job's `sidekiq_options`. The threshold is capped at the job's final attempt. Not set by default.
+
+```ruby
+# Report only the last of 5 attempts (1 initial execution + 4 retries)
+sidekiq_options retry: 4, attempt_threshold: 5
+```