[DOC] Improve pc.Application API ref docs.
[DOC] Improve pc.Application API ref docs.

playcanvas/engine,sereepap2029/playcanvas,MicroWorldwide/PlayCanvas,MicroWorldwide/PlayCanvas,aidinabedi/playcanvas-engine,sereepap2029/playcanvas,aidinabedi/playcanvas-engine,sereepap2029/playcanvas,playcanvas/engine