make wording around duplicate tracestate headers more precise

Participants must not add duplicated tracestate keys, but they are not
responsible for cleaning up duplicated tracestate keys that another
updstream participant has added.

fixes #548

spec/20-http_request_header_format.md

@@ -318,7 +318,7 @@ The `tracestate` value is the concatenation of trace graph key/value pairs.
 
 Example: `vendorname1=opaqueValue1,vendorname2=opaqueValue2`
 
-Only one entry per key is allowed. For example, if a vendor name is Congo and a trace started in their system and then went through a system named Rojo and later returned to Congo, the `tracestate` value would not be:
+Tracing tools are not supposed to add the same header multiple times. For example, if a vendor name is Congo and a trace started in their system and then went through a system named Rojo and later returned to Congo, the `tracestate` value would not be:
 
 `congo=congosFirstPosition,rojo=rojosFirstPosition,congo=congosSecondPosition`