Add docstring and comment for default initializer
Add docstring and comment for default initializer

ktnyt/chainer,rezoo/chainer,delta2323/chainer,cupy/cupy,jnishi/chainer,hvy/chainer,hvy/chainer,cupy/cupy,ysekky/chainer,anaruse/chainer,niboshi/chainer,ktnyt/chainer,okuta/chainer,ronekko/chainer,hvy/chainer,chainer/chainer,keisuke-umezawa/chainer,kashif/chainer,keisuke-umezawa/chainer,tkerola/chainer,chainer/chainer,niboshi/chainer,keisuke-umezawa/chainer,ktnyt/chainer,pfnet/chainer,cupy/cupy,kiyukuta/chainer,okuta/chainer,jnishi/chainer,wkentaro/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer,aonotas/chainer,okuta/chainer,jnishi/chainer,chainer/chainer,cupy/cupy,ktnyt/chainer,jnishi/chainer,wkentaro/chainer,keisuke-umezawa/chainer,okuta/chainer