doc(core): Fix outdated JSDoc in beforeSendSpan

[Lms24]

Spotted that we still documented that the root span does not go through beforeSendSpan. This is outdated. Since v9, we also pass the root span to the callback.

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.
⚠️ Warning: Base artifact is not the latest one, because the latest workflow run is not done yet. This may lead to incorrect results. Try to re-run all tests to get up to date results.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	8,369	-	9,189	-9%
GET With Sentry	1,326	16%	1,441	-8%
GET With Sentry (error only)	5,902	71%	6,134	-4%
POST Baseline	1,184	-	1,223	-3%
POST With Sentry	496	42%	561	-12%
POST With Sentry (error only)	1,039	88%	1,078	-4%
MYSQL Baseline	3,256	-	3,370	-3%
MYSQL With Sentry	361	11%	503	-28%
MYSQL With Sentry (error only)	2,617	80%	2,778	-6%

View base workflow run