Added VS attrib doc.
Added VS attrib doc.

LSBOSS/bgfx,aonorin/bgfx,MikePopoloski/bgfx,MikePopoloski/bgfx,MikePopoloski/bgfx,marco-we/bgfx,cuavas/bgfx,mcanthony/bgfx,kondrak/bgfx,fluffyfreak/bgfx,ktotheoz/bgfx,LWJGL-CI/bgfx,cyndis/bgfx,ocornut/bgfx,darkimage/bgfx,LWJGL-CI/bgfx,mendsley/bgfx,attilaz/bgfx,fluffyfreak/bgfx,darkimage/bgfx,mcanthony/bgfx,ming4883/bgfx,LWJGL-CI/bgfx,sergeScherbakov/bgfx,janstk/bgfx,ktotheoz/bgfx,jdryg/bgfx,elmindreda/bgfx,LSBOSS/bgfx,elmindreda/bgfx,v3n/bgfx,Vertexwahn/bgfx,bkaradzic/bgfx,emoon/bgfx,jpcy/bgfx,bkaradzic/bgfx,bkaradzic/bgfx,jpcy/bgfx,fluffyfreak/bgfx,andr3wmac/bgfx,ocornut/bgfx,Extrawurst/bgfx,Vertexwahn/bgfx,andr3wmac/bgfx,darkimage/bgfx,elmindreda/bgfx,attilaz/bgfx,0-wiz-0/bgfx,ktotheoz/bgfx,jdryg/bgfx,BlueCrystalLabs/bgfx,marco-we/bgfx,0-wiz-0/bgfx,jdryg/bgfx,ocornut/bgfx,mcanthony/bgfx,fluffyfreak/bgfx,Synxis/bgfx,mmicko/bgfx,v3n/bgfx,emoon/bgfx,BlueCrystalLabs/bgfx,Extrawurst/bgfx,attilaz/bgfx,marco-we/bgfx,andr3wmac/bgfx,jpcy/bgfx,jdryg/bgfx,cyndis/bgfx,Vertexwahn/bgfx,kondrak/bgfx,mmicko/bgfx,janstk/bgfx,sergeScherbakov/bgfx,LWJGL-CI/bgfx,cuavas/bgfx,cyndis/bgfx,emoon/bgfx,BlueCrystalLabs/bgfx,ming4883/bgfx,mmicko/bgfx,aonorin/bgfx,bkaradzic/bgfx,Extrawurst/bgfx,cuavas/bgfx,aonorin/bgfx,septag/bgfx,janstk/bgfx,v3n/bgfx,septag/bgfx,mendsley/bgfx,Synxis/bgfx,ming4883/bgfx,mendsley/bgfx,LSBOSS/bgfx,jpcy/bgfx,sergeScherbakov/bgfx,septag/bgfx,Synxis/bgfx,0-wiz-0/bgfx,kondrak/bgfx