clarifies parent and links (open-telemetry#283)

* clarifies parent and links

* Update specification/overview.md

specification/overview.md

@@ -95,12 +95,23 @@ propagated from parent to child **Spans**.
 A **Span** may be linked to zero or more other **Spans** (defined by
 **SpanContext**) that are causally related. **Links** can point to
 **SpanContexts** inside a single **Trace** or across different **Traces**.
-**Links** can be used to represent batched operations where a **Span** has
-multiple parents, each representing a single incoming item being processed in
-the batch. Another example of using a **Link** is to declare relationship
-between originating and restarted trace. This can be used when **Trace** enters
-trusted boundaries of an service and service policy requires to generate a new
-Trace instead of trusting incoming Trace context. 
+**Links** can be used to represent batched operations where a **Span** was
+initiated by multiple initiating **Span**s, each representing a single incoming
+item being processed in the batch. Another example of using a **Link** is to
+declare relationship between originating and followed trace. This can be used
+when **Trace** enters trusted boundaries of a service and service policy
+requires to generate a new Trace instead of trusting incoming Trace context. Or
+when long running Trace representing asynchronous data processing operation was
+initiated by one of many fast incoming request.
+
+In case of scatter/gather pattern, when the root operation starts multiple
+downstream processing operations and all of them being aggregated back in a
+single **Span**, this last **Span** is linked to many operations it
+aggregates. All of them are the **Span**s from the same Trace. And similar to
+the Parent field of a **Span**. It is recommended, however, to not set parent of
+the **Span** in this scenario as semantically parent field represents a single
+parent scenario, in many cases parent **Span** fully encloses the child
+**Span**. Which is not the case in scatter/gather and batch scenarios.
 
 ## Metrics
 

specification/sdk-tracing.md

@@ -46,7 +46,8 @@ Returns the sampling Decision for a `Span` to be created.
 - `SpanId` of the `Span` to be created.
 - Name of the `Span` to be created.
 - Collection of links that will be associated with the `Span` to be created.
-  Typically useful for batch operations.
+  Typically useful for batch operations, see [Links Between
+  Spans](overview.md#links-between-spans).
 
 **Return value:**