feat(js): Add documentation for setActiveSpanInBrowser(span: Span) (#15007)

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## DESCRIBE YOUR PR

Adds documentation for `Sentry. setActiveSpanInBrowser(span)` which is
added via getsentry/sentry-javascript#17714. To
be merged only after the feature was released. This is still based on
#15006 because I need the `availableSince` prop on the `SdkApi`
component.

closes https://linear.app/getsentry/issue/FE-608/docs-for-setspanactive

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## PRE-MERGE CHECKLIST

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## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

---------

Co-authored-by: Shannon Anahata <shannon.anahata@gmail.com>

Author: Lms24

docs/platforms/javascript/common/apis.mdx

@@ -636,6 +636,43 @@ See <PlatformLink to="/tracing/instrumentation/">Tracing Instrumentation</Platfo
 
 </SdkApi>
 
+<SdkApi
+ name="setActiveSpanInBrowser"
+ signature="function setActiveSpanInBrowser(span: Span): void"
+ availableSince="10.15.0"
+ categorySupported={["browser"]}
+ >
+  Sets the passed span as the active span on the current scope.
+  You most likely don't need this functionality and should [use `startSpan`](#startSpan) 
+  instead.
+  However, in some situations, you might prefer a span being active outside of a callback. 
+  In this case, the combination of [`startInactiveSpan`](#startInactiveSpan) with this function
+  allows you to start a span, hold a reference to it and keep it active until you end it, without
+  the active duration being bound to the callback (as opposed to [`startSpanManual`](#startSpanManual)).
+  
+  
+  <Expandable title='Examples'>
+  ```javascript
+  let checkoutSpan;
+  
+  on('startCheckout', () => {
+    checkoutSpan = Sentry.startInactiveSpan({name: 'checkout-flow'});
+    Sentry.setActiveSpanInBrowser(checkoutSpan);
+  })
+  
+  doSomeWork();
+  
+  on('endCheckout', () => {
+    // Ending the span automatically removes it as the active span on the scope
+    checkoutSpan.end();
+  })
+  ```
+</Expandable>
+
+See <PlatformLink to="/tracing/instrumentation/">Tracing Instrumentation</PlatformLink> for more information on how to work with spans.
+
+</SdkApi>
+
 <SdkApi
   name="continueTrace"
   signature="function continueTrace<T>(options: TraceOptions, callback: () => T): T"

docs/platforms/javascript/common/tracing/instrumentation/index.mdx

@@ -102,8 +102,46 @@ Sometimes, you do not want the span to be ended automatically when the callback
 
 To add spans that aren't active, you can create independent spans. This is useful when you have work that is grouped together under a single parent span, but is independent from the currently active span. However, in most cases you'll want to create and use the [startSpan](#starting-an-active-span-startspan) API from above.
 
+
 <PlatformContent includePath="performance/start-inactive-span" />
 
+<PlatformCategorySection supported={['browser']}>
+
+  ### Setting an inactive span active (browser only)
+  
+  _Available Since: `10.15.0`_
+  
+  In browser environments, you might run into use cases, where the callback-based span APIs are not sufficient.
+  In such cases (see example below), you can use `startInactiveSpan` to start an initially inactive span and then 
+  set it active until you manually end the span.
+
+  ```javascript
+  let checkoutSpan;
+  
+  on('startCheckout', () => {
+    checkoutSpan = Sentry.startInactiveSpan({name: 'checkout-flow'});
+    Sentry.setActiveSpanInBrowser(checkoutSpan);
+  })
+  
+  doSomeWork();
+  
+  on('endCheckout', () => {
+    // Ending the span automatically removes it as the active span
+    checkoutSpan.end();
+  })
+  ```
+  
+  Using `startInactiveSpan` in combination with `setActiveSpanInBrowser` allows you to create an inactive span that can be made active until you manually end it. In contrast, [`startSpanManual`](#starting-an-active-span-with-manual-end-startspanmanual) allows you to end the span manually at any point, but it only remains active within the callback.
+  
+  <Alert>
+  
+  Note that `setActiveSpanInBrowser` is only available in browser environments! If you're running code in the server
+  (for example for server-side rendering), make sure to guard this call to only run in the browser.
+  
+  </Alert>
+
+</PlatformCategorySection>
+
 ## Starting Spans as Children of a Specific Span
 
 By default, any span that is started will be the child of the currently active span. If you want to have a different behavior, you can force spans to be the children of a specific span with the `parentSpan` option: