Make clear SupportsRequestPriority() also implies connection sharing.

BUG=637407
R=rch@chromium.org

Review-Url: https://codereview.chromium.org/2286373002
Cr-Commit-Position: refs/heads/master@{#415222}

net/http/http_server_properties.h

@@ -234,6 +234,9 @@ class NET_EXPORT HttpServerProperties {
 
   // Returns true if |server| supports a network protocol which honors
   // request prioritization.
+  // Note that this also implies that the server supports request
+  // multiplexing, since priorities imply a relationship between
+  // multiple requests.
   virtual bool SupportsRequestPriority(const url::SchemeHostPort& server) = 0;
 
   // Returns the value set by SetSupportsSpdy(). If not set, returns false.