BufGrain automatically supplies an envelope

[woolgathering]

Essentially BufGrain supplies an envelope without mentioning it in help. Figured it would be useful to update the helpfile so users know.

a = Array.fill(44100, {rrand(-1.0, 1.0)}); // noise
b = Buffer.loadCollection(s, a); // make it a buffer
{BufGrain.ar(Impulse.kr(0.5), 1, b) * 0.2}.play; // automatic sine window