[trace-sdk-sampler] Differentiate between root and child span creation in trace sampling spec

specification/trace/sdk.md

@@ -295,9 +295,13 @@ When asked to create a Span, the SDK MUST act as if doing the following in order
    (Note that the [built-in `ParentBasedSampler`](#parentbased) can be used to
    use the sampling decision of the parent,
    translating a set SampledFlag to RECORD and an unset one to DROP).
-3. Generate a new span ID for the `Span`, independently of the sampling decision.
-   This is done so other components (such as logs or exception handling) can rely on
-   a unique span ID, even if the `Span` is a non-recording instance.
+3. If there isn't currently a span (i.e the span being created will become the
+   root span), SDKs MUST generate a new span ID for the `Span`, independently of
+   the sampling decision. This is done so other components (such as logs or
+   exception handling) can rely on a unique span ID, even if the `Span` is a
+   non-recording instance. For non-root spans, SDKs MAY skip creation of spans
+   if `ShouldSample` results in a non-recording instance (`DROP`). This is done
+   to optimize the performance by avoiding creating a span when unnecessary.
 4. Create a span depending on the decision returned by `ShouldSample`:
    see description of [`ShouldSample`'s](#shouldsample) return value below
    for how to set `IsRecording` and `Sampled` on the Span,
@@ -307,6 +311,10 @@ When asked to create a Span, the SDK MUST act as if doing the following in order
    `Span` is created without an SDK installed or as described in
    [wrapping a SpanContext in a Span](api.md#wrapping-a-spancontext-in-a-span).
 
+Note: Once the span creation behavior has shipped in a stable SDK release it
+MUST NOT be changed without clear indication of a breaking change being made
+(major version bump typically).
+
 ### Sampler
 
 `Sampler` interface allows users to create custom samplers which will return a