Rollup merge of #86389 - kpreid:sum, r=scottmcm
Rollup merge of #86389 - kpreid:sum, r=scottmcm

Make `sum()` and `product()` documentation hyperlinks refer to `Iterator` methods.

The previous linking seemed confusing: within "the sum() method on iterators", "sum()" was linked to `Sum::sum`, not `Iterator::sum`, even though the sentence is talking about the latter. I have rewritten the sentence to be, I believe, clearer, as well as changing the link destinations; applying the same change to the `Product` documentation as well as `Sum`.

I reviewed other traits in the same module and did not see similar issues, and previewed the results using `./x.py doc library/std`.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust