Apply suggestions from code review

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

specification/trace/sdk.md

@@ -197,9 +197,9 @@ Thus, the SDK specification defines sets of possible requirements for
 
 ## TraceState manipulation
 
-`TraceState` is a part of [`SpanContext`](./api.md#spancontext), represented by a list of key-values pairs and
+`TraceState` is a part of [`SpanContext`](./api.md#spancontext), represented by a list of string key-values pairs and
  formally defined by [W3C specification](https://www.w3.org/TR/trace-context/#tracestate-header).
-Tracing SDK MUST provide at least the following mutation operations on `TraceState`:
+Tracing SDK MUST provide at least the following operations on `TraceState` which return a new `TraceState` with the modifications applied:
 
 * Update key value
 * Add a new key/value pair
@@ -211,7 +211,7 @@ Please note, that as `SpanContext` is immutable, it is not possible to update `S
 Such changes then make sense only right before 
 [`SpanContext` propagation](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md) 
 or [telemetry data exporting](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#span-exporter).
-In both cases `Propagators` and `SpanExporters` may modify `TraceState` before serializing it to the wire.
+In both cases `Propagators` and `SpanExporters` may create a modified `TraceState` copy before serializing it to the wire.
 
 ## Span processor