Rollup merge of #88523 - kpreid:category, r=yaahc

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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I considered making similar changes to the documentation of the `is_*` methods of floats, but decided that that was a much larger and trickier problem; here, each of the variants' descriptions can be expected to be read in context of being mutually exclusive with the others.

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