Improve RandomNumberGenerator docs.
Improve RandomNumberGenerator docs.

Clarified behavior of a few methods. Added implementation notes so
that developers not familiar with RNGs are alerted to common pitfalls.

pkowal1982/godot,josempans/godot,Paulloz/godot,sanikoyes/godot,guilhermefelipecgs/godot,groud/godot,BastiaanOlij/godot,Shockblast/godot,vkbsb/godot,sanikoyes/godot,Faless/godot,Zylann/godot,BastiaanOlij/godot,Shockblast/godot,Shockblast/godot,guilhermefelipecgs/godot,josempans/godot,BastiaanOlij/godot,josempans/godot,honix/godot,MarianoGnu/godot,honix/godot,Valentactive/godot,Paulloz/godot,Faless/godot,ZuBsPaCe/godot,vkbsb/godot,Faless/godot,godotengine/godot,firefly2442/godot,firefly2442/godot,ZuBsPaCe/godot,pkowal1982/godot,groud/godot,akien-mga/godot,Zylann/godot,vnen/godot,godotengine/godot,honix/godot,sanikoyes/godot,ZuBsPaCe/godot,Faless/godot,okamstudio/godot,Paulloz/godot,Faless/godot,ex/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,sanikoyes/godot,akien-mga/godot,okamstudio/godot,firefly2442/godot,vnen/godot,okamstudio/godot,vnen/godot,akien-mga/godot,BastiaanOlij/godot,guilhermefelipecgs/godot,Valentactive/godot,Faless/godot,BastiaanOlij/godot,vnen/godot,vkbsb/godot,ZuBsPaCe/godot,okamstudio/godot,Paulloz/godot,Shockblast/godot,Valentactive/godot,firefly2442/godot,DmitriySalnikov/godot,pkowal1982/godot,groud/godot,ex/godot,Zylann/godot,Paulloz/godot,MarianoGnu/godot,DmitriySalnikov/godot,godotengine/godot,okamstudio/godot,Faless/godot,Valentactive/godot,pkowal1982/godot,josempans/godot,Valentactive/godot,pkowal1982/godot,MarianoGnu/godot,honix/godot,honix/godot,ex/godot,godotengine/godot,sanikoyes/godot,vkbsb/godot,josempans/godot,sanikoyes/godot,Shockblast/godot,groud/godot,Valentactive/godot,vnen/godot,guilhermefelipecgs/godot,Zylann/godot,ex/godot,okamstudio/godot,ex/godot,Zylann/godot,Shockblast/godot,MarianoGnu/godot,ex/godot,sanikoyes/godot,sanikoyes/godot,MarianoGnu/godot,Zylann/godot,guilhermefelipecgs/godot,MarianoGnu/godot,DmitriySalnikov/godot,Faless/godot,pkowal1982/godot,Shockblast/godot,guilhermefelipecgs/godot,honix/godot,Zylann/godot,vkbsb/godot,okamstudio/godot,vnen/godot,Shockblast/godot,ex/godot,akien-mga/godot,firefly2442/godot,akien-mga/godot,Valentactive/godot,josempans/godot,ZuBsPaCe/godot,godotengine/godot,ZuBsPaCe/godot,BastiaanOlij/godot,ZuBsPaCe/godot,josempans/godot,godotengine/godot,MarianoGnu/godot,pkowal1982/godot,vnen/godot,MarianoGnu/godot,akien-mga/godot,Zylann/godot,groud/godot,vkbsb/godot,BastiaanOlij/godot,firefly2442/godot,groud/godot,Paulloz/godot,okamstudio/godot,pkowal1982/godot,firefly2442/godot,guilhermefelipecgs/godot,akien-mga/godot,firefly2442/godot,akien-mga/godot,Paulloz/godot,vnen/godot,BastiaanOlij/godot,vkbsb/godot,DmitriySalnikov/godot,godotengine/godot,godotengine/godot,ex/godot,DmitriySalnikov/godot,Valentactive/godot,josempans/godot,okamstudio/godot,guilhermefelipecgs/godot,okamstudio/godot,vkbsb/godot,DmitriySalnikov/godot