docs: trace sampling probability (#5129)

docs/source/configuration/telemetry/apollo-telemetry.mdx

@@ -34,16 +34,17 @@ Your subgraph libraries must support federated tracing (also known as FTV1 traci
 - Consult your library's documentation to learn how to enable federated tracing.
     - If you use Apollo Server with `@apollo/subgraph`, federated tracing support is enabled automatically.
 
-### Trace sampling rate
+### Subgraph trace sampling
 
-By default, the Apollo Router requests subgraph trace data for 1% of operations. In most cases, this provides a sufficient sample size while minimizing latency for most operations (traces can affect latency because they increase the size of subgraph response payloads).
+By default, the Apollo Router requests subgraph trace data from operations with a 1% sampling probability per operation. In most cases, this provides a sufficient sample size while minimizing latency for most operations (traces can affect latency because they increase the size of subgraph response payloads).
 
-You can customize your router's trace sampling rate by setting the following options in your [YAML config file](./overview/#yaml-config-file):
+You can customize your router's trace sampling probability by setting the following options in your [YAML config file](./overview/#yaml-config-file):
 
 ```yaml title="router.yaml"
 telemetry:
   apollo:
-    # In this example, the router will request traces for 50% of requests.
+    # In this example, the trace sampler is configured  
+    # with a 50% probability of sampling a request.
     # This value can't exceed the value of tracing.common.sampler.
     field_level_instrumentation_sampler: 0.5