Update SpanKind comments (#64)

This brings the improved comments from open-telemetry/oteps#59

opentelemetry/proto/trace/v1/trace.proto

@@ -98,31 +98,32 @@ message Span {
   // This field is required.
   string name = 7;
 
-  // Type of span. Can be used to specify additional relationships between spans
+  // SpanKind is the type of span. Can be used to specify additional relationships between spans
   // in addition to a parent/child relationship.
   enum SpanKind {
     // Unspecified. Do NOT use as default.
     // Implementations MAY assume SpanKind to be INTERNAL when receiving UNSPECIFIED.
     SPAN_KIND_UNSPECIFIED = 0;
 
-    // Indicates that the span is used internally. Default value.
+    // Indicates that the span represents an internal operation within an application,
+    // as opposed to an operations happening at the boundaries. Default value.
     INTERNAL = 1;
 
     // Indicates that the span covers server-side handling of an RPC or other
     // remote network request.
     SERVER = 2;
 
-    // Indicates that the span covers the client-side wrapper around an RPC or
-    // other remote request.
+    // Indicates that the span describes a request to some remote service.
     CLIENT = 3;
 
-    // Indicates that the span describes producer sending a message to a broker.
-    // Unlike client and  server, there is no direct critical path latency relationship
-    // between producer and consumer spans.
+    // Indicates that the span describes a producer sending a message to a broker.
+    // Unlike CLIENT and SERVER, there is often no direct critical path latency relationship
+    // between producer and consumer spans. A PRODUCER span ends when the message was accepted
+    // by the broker while the logical processing of the message might span a much longer time.
     PRODUCER = 4;
 
-    // Indicates that the span describes consumer recieving a message from a broker.
-    // Unlike client and  server, there is no direct critical path latency relationship
+    // Indicates that the span describes consumer receiving a message from a broker.
+    // Like the PRODUCER kind, there is often no direct critical path latency relationship
     // between producer and consumer spans.
     CONSUMER = 5;
   }