Rollup merge of #95483 - golddranks:improve_float_docs, r=joshtriplett

Improve floating point documentation

This is my attempt to improve/solve https://github.com/rust-lang/rust/issues/95468 and https://github.com/rust-lang/rust/issues/73328 .

Added/refined explanations:
- Refine the "NaN as a special value" top level explanation of f32
- Refine `const NAN` docstring: add an explanation about there being multitude of NaN bitpatterns and disclaimer about the portability/stability guarantees.
- Refine `fn is_sign_positive` and `fn is_sign_negative` docstrings: add disclaimer about the sign bit of NaNs.
- Refine `fn min` and `fn max` docstrings: explain the semantics and their relationship to the standard and libm better.
- Refine `fn trunc` docstrings: explain the semantics slightly more.
- Refine `fn powi` docstrings: add disclaimer that the rounding behaviour might be different from `powf`.
- Refine `fn copysign` docstrings: add disclaimer about payloads of NaNs.
- Refine `minimum` and `maximum`: add disclaimer that "propagating NaN" doesn't mean that propagating the NaN bit patterns is guaranteed.
- Refine `max` and `min` docstrings: add "ignoring NaN" to bring the one-row explanation to parity with `minimum` and `maximum`.

Cosmetic changes:
- Reword `NaN` and `NAN` as plain "NaN", unless they refer to the specific `const NAN`.
- Reword "a number" to `self` in function docstrings to clarify.
- Remove "Returns NAN if the number is NAN" from `abs`, as this is told to be the default behavior in the top explanation.

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