Update status serialization for Zipkin (was: Remove mention of gRPC s…

…tatus codes) (open-telemetry#1186)

Co-authored-by: Christian Neumüller <[email protected]>

specification/trace/sdk_exporters/zipkin.md

@@ -138,18 +138,16 @@ TBD: add examples
 
 ### Status
 
-Span `Status` MUST be reported as a key-value pair in `tags` to Zipkin.
+Span `Status` MUST be reported as a key-value pair in `tags` to Zipkin, unless it is `UNSET`.
+In the latter case it MUST NOT be reported.
 
 The following table defines the OpenTelemetry `Status` to Zipkin `tags` mapping.
 
 | Status|Tag Key| Tag Value |
 |--|--|--|
-|Code | `otel.status_code` | Name of the code, for example: `OK` |
+|Code | `otel.status_code` | Name of the code, either `OK` or `ERROR`. MUST NOT be set if the code is `UNSET`. |
 |Message *(optional)* | `otel.status_description` | `{message}` |
 
-The `otel.status_code` tag value MUST follow the [Standard GRPC Code
-Names](https://github.com/grpc/grpc/blob/master/doc/statuscodes.md).
-
 ### Events
 
 OpenTelemetry `Event` has an optional `Attribute`(s) which is not supported by