[jaeger exporter] Document mapping of service name (#1222)

CHANGELOG.md

@@ -9,6 +9,8 @@ release.
 
 New:
 
+- Document service name mapping for Jaeger exporters
+  ([1222](https://github.com/open-telemetry/opentelemetry-specification/pull/1222))
 - Add performance benchmark specification
   ([#748](https://github.com/open-telemetry/opentelemetry-specification/pull/748))
 - Enforce that the Baggage API must be fully functional, even without an installed SDK.

specification/trace/sdk_exporters/jaeger.md

@@ -18,9 +18,13 @@ and Jaeger.
 
 ### Resource
 
-OpenTelemetry resources MUST be mapped to Jaeger process tags. Multiple resources can exist for a
+OpenTelemetry resources MUST be mapped to Jaeger's `Span.Process` tags. Multiple resources can exist for a
 single process and exporters need to handle this case accordingly.
 
+Critically, Jaeger backend depends on `Span.Process.ServiceName` to identify the service
+that produced the spans. That field MUST be populated from the `service.name` attribute
+of the [`service` resource](../../resource/semantic_conventions/README.md#service).
+
 ### InstrumentationLibrary
 
 OpenTelemetry Span's `InstrumentationLibrary` MUST be reported as span `tags` to Jaeger using the following mapping.