Clarify doc comment for DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED (#6791)

## Summary of changes
Small docs update.

## Reason for change
Adds more clarity.

## Implementation details

## Test coverage
N/A

## Other details

---------

Co-authored-by: Andrew Lock <andrew.lock@datadoghq.com>

Author: zacharycmontoya

tracer/src/Datadog.Trace/Configuration/ConfigurationKeys.cs

@@ -805,9 +805,11 @@ internal static class FeatureFlags
             /// If disabled, 128-bit trace ids will be truncated to the lower 64 bits,
             /// and injected as decimal strings. 64-bit trace ids are
             /// always injected as decimal strings, regardless of this setting.
-            /// Default value is <c>true</c> (enabled).
-            /// If <see cref="ConfigurationKeys.FeatureFlags.TraceId128BitGenerationEnabled"/> is set to <c>false</c>,
-            /// this setting will also be set to <c>false</c>.
+            /// If unset, this configuration will take the value of the <see cref="ConfigurationKeys.FeatureFlags.TraceId128BitGenerationEnabled"/> configuration,
+            /// which is <c>true</c> by default.
+            /// Note: This configuration can be set independently of the <see cref="ConfigurationKeys.FeatureFlags.TraceId128BitGenerationEnabled"/> configuration,
+            /// so it's possible to inject 128-bit trace ids into logs even if the application is only generating 64-bit trace ids, since distributed traces from upstream
+            /// services may contain 128-bit trace ids.
             /// </summary>
             public const string TraceId128BitLoggingEnabled = "DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED";