feat(js): Add docs for updateSpanName

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

@@ -235,16 +235,22 @@ Sentry maintains a [list of well known span operations](https://develop.sentry.d
 
 ### Updating the Span Name
 
+_Available since: v8.39.0_
+
 You can update the name of a span at any time:
 
 ```javascript
 const span = Sentry.getActiveSpan();
 if (span) {
-  span.updateName("New Name");
+  Sentry.updateSpanName(span, "New Name");
 }
 ```
 
-Please note that in certain scenarios, the span name will be overwritten by the SDK. This is the case for spans with any of the following attribute combinations:
+Prior to v8.39.0, you had to use `span.updateName('New Name')`, which had some limitations in `@sentry/node` and SDKs depending on it (e.g. `@sentry/nextjs`):
+
+- Spans with `http.method` or `http.request.method` attributes would automatically have their name set to the method + the URL path
+- Spans with `db.system` attributes would automatically have their name set to the system + the statement
+
+Using `Sentry.updateSpanName()` instead of `span.updateName()` ensures that the name is updated correctly and no longer overwritten in all cases.
 
-* Spans with `http.method` or `http.request.method` attributes will automatically have their name set to the method + the URL path
-* Spans with `db.system` attributes will automatically have their name set to the system + the statement
+In browser environments (if you use `@sentry/browser`, `@sentry/react`, etc), `span.updateName()` and `Sentry.updateSpanName()` are identical and you can use either of them.