Clarify docstring for `exponent`, again (#47085)
In #47035 we realised that `issubnormal` used the word "exponent" quite
loosely, and this is also true of `exponent`'s docstring. So this tries
to fix it...
* Saying "this corresponds to the exponent of `x`." seems a bit
circular, but I think it wants to say that in such cases it corresponds
to the exponent bits?
* Saying "normalized floating-point number" does not mean like
`normalize(1.0)`... I think it means not-subnormal. Maybe it's better to
say that? (Although I don't love this sentence.)
* From "largest integer y such that `2^y ≤ abs(x)`" it sounds like
`exponent(8+im)` should work, but it does not. So I marked it `::Real`.
* I think it is nice to mention that 0, Inf, Nan give an error. As they
must, for `2^y ≤ abs(x)`... but `0.0` is not subnormal, so the claim
about reading the bit pattern isn't quite correct without excluding
these cases.
* Better examples, progressively adding detail, not too repetitive.
* See also related functions.
Recently edited in #46815.
And I forgot, but "See also" and errors also overlap with this
(approved) PR:
https://github.com/JuliaLang/julia/pull/45221/files#diff-faa5f0835bd28c4a33e6603e208a9236034568a4abc7adec4995afb64550d0bdR940
. That should go first.
