ref(tracing): Update long task description

[AbhiPrasad]

Update the ui.long-task span description to explain what is happening while the span is occuring.

https://developer.mozilla.org/en-US/docs/Web/API/Long_Tasks_API

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.4 KB (-0.02% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.06 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.98 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	52.92 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.77 KB (0%)
@sentry/browser - Webpack (minified)	64.31 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.79 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.71 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.91 KB (+0.07% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.28 KB (+0.05% 🔺)

[k-fish]

I think the op + the description will help this be clearer, especially since it uses the same terminology as seen on the mdn docs etc. We can potentially add an explanation in the UI if we get questions about this and link out to docs.