Missing documentations of overloaded Variable operators

[hvy]

Overloaded operators for the Variable class is not documented.

Some are taken for granted such as +, -, but others like *, @ and abs() might be less obvious to new users.

List of methods
https://github.com/chainer/chainer/blob/master/chainer/functions/math/basic_math.py#L806-L822

[stale]

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[hvy]

bump