Expand documentation for `FpCategory`.

I intend these changes to be helpful to readers who are not yet familiar
with the quirks of floating-point numbers. Additionally, I felt it was
misleading to describe `Nan` as being the result of division by zero,
since most divisions by zero (except for 0/0) produce `Infinite` floats,
so I moved that remark to the `Infinite` variant with adjustment.

The first sentence of the `Nan` documentation is copied from `f32`;
I followed the example of the `f64` documentation by referring to `f32`
for general concepts, rather than duplicating the text.

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