feat(js): Improve tracesSampler docs for JS (#10759)

Author: mydea

docs/platforms/javascript/common/configuration/sampling.mdx

@@ -58,18 +58,12 @@ By default, none of these options are set, meaning no transactions will be sent
 
 ## Sampling Context Data
 
-### Default Sampling Context Data
-
-The information contained in the <PlatformIdentifier name="sampling-context" /> object passed to the <PlatformIdentifier name="traces-sampler" /> when a transaction is created varies by platform and integration.
-
-<PlatformContent includePath="performance/default-sampling-context" />
-
-### Custom Sampling Context Data
-
-When using custom instrumentation to create a span, the `tracesSampler` function will automatically receive the span `name` and the initial span `attributes` to use in its sampling decision.
+When a span is started, the `tracesSampler` function will automatically receive the span `name` and the initial span `attributes` to use in its sampling decision. Additionally, it will also receive a `parentSampled` boolean indicating whether the parent span was sampled. This data can be used to make a more informed decision about whether to sample the span.
 
 <PlatformContent includePath="performance/custom-sampling-context" />
 
+Please note that the `name` passed to `tracesSampler` may not be the exact same name that is eventually sent to Sentry. The name may be updated during the lifetime of the span, but the `name` passed to `tracesSampler` will always be the initial name. For example, the `name` for `http.server` spans will usually not be parametrized yet when passed to `tracesSampler`, so instead of `GET /users/:id` you may see a name of `GET /users/123`.
+
 </PlatformSection>
 
 ## Inheritance