specs: Add link to D3DPERF_* reference.
specs: Add link to D3DPERF_* reference.

Not easy to find after Microsoft integrated DXSDK into Windows SDK and
wiped out all DXSDK docs from MSDN...

schulmar/apitrace,swq0553/apitrace,trtt/apitrace,swq0553/apitrace,EoD/apitrace,swq0553/apitrace,schulmar/apitrace,EoD/apitrace,joshua5201/apitrace,tuanthng/apitrace,schulmar/apitrace,trtt/apitrace,EoD/apitrace,schulmar/apitrace,tuanthng/apitrace,trtt/apitrace,tuanthng/apitrace,apitrace/apitrace,tuanthng/apitrace,swq0553/apitrace,surround-io/apitrace,swq0553/apitrace,EoD/apitrace,surround-io/apitrace,surround-io/apitrace,apitrace/apitrace,surround-io/apitrace,schulmar/apitrace,trtt/apitrace,trtt/apitrace,apitrace/apitrace,joshua5201/apitrace,surround-io/apitrace,apitrace/apitrace,EoD/apitrace,tuanthng/apitrace,joshua5201/apitrace,joshua5201/apitrace,joshua5201/apitrace