document combination of repeated response headers

docs/user/quickstart.rst

@@ -347,6 +347,15 @@ So, we can access the headers using any capitalization we want::
     >>> r.headers.get('content-type')
     'application/json'
 
+It is also special in that the server could have sent the same header multiple
+times with different values, but requests combines them so they can be
+represented in the dictionary within a single mapping, as per
+`RFC 7230 <http://tools.ietf.org/html/rfc7230#section-3.2>`_:
+
+    > A recipient MAY combine multiple header fields with the same field name
+    > into one "field-name: field-value" pair, without changing the semantics
+    > of the message, by appending each subsequent field value to the combined
+    > field value in order, separated by a comma.
 
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