doc(core): Improve beforeSendSpan JSDoc (#13131)

This PR changes the `beforeSendSpan` JSDoc analogously to the docs
change (getsentry/sentry-docs#10907).
It now more clearly points out that `beforeSendSpan` is only called for
child spans but not for root spans.


ref #13119

packages/types/src/options.ts

@@ -286,11 +286,15 @@ export interface ClientOptions<TO extends BaseTransportOptions = BaseTransportOp
   beforeSend?: (event: ErrorEvent, hint: EventHint) => PromiseLike<ErrorEvent | null> | ErrorEvent | null;
 
   /**
-   * An event-processing callback for spans. This allows a span to be modified before it's sent.
-   *
+   * This function can be defined to modify or entirely drop a child span before it's sent.
    * Returning `null` will cause this span to be dropped.
+   *
+   * Note that this function is only called for child spans and not for the root span (formerly known as transaction).
+   * If you want to modify or drop the root span, use {@link Options.beforeSendTransaction} instead.
+   *
    * @param span The span generated by the SDK.
-   * @returns A new span that will be sent | null.
+   *
+   * @returns A new span that will be sent or null if the span should not be sent.
    */
   beforeSendSpan?: (span: SpanJSON) => SpanJSON | null;