Update docs for Fractional, Algebraic, Round, and Trigonometric traits.
Add docs for traits Exponential, Hyperbolic, BitCount.
Got directions backwards in to_degrees and to_radians docs.
Use constants instead of raw values.
auto merge of #10435 : jayanderson/rust/docs, r=huonw

This fills in some missing docs in the nums package. Let me know if this is on the right track for what's wanted for docs. I can probably fill in more in the future. Thanks.

(As a side note the precedence of the unary negative operator '-' tripped me up for a bit. Essentially I would expect `-25.0f32.sqrt()` to result in NaN instead of `-5.0`.)