Rewrite the first sentence in slice::sort

For every method, the first sentence should consisely explain what it does,
not how. This sentence usually starts with a verb.

It's really weird for `sort` to be explained in terms of another function,
namely `sort_by`. There's no need for that because it's obvious how `sort`
sorts elements: there is `T: Ord`.

If `sort_by_key` does not have to explicitly state how it's implemented,
then `sort` doesn't either.

r? @steveklabnik

Rollup of 21 pull requests

- Successful merges: #38617, #39284, #39285, #39290, #39302, #39305, #39306, #39307, #39311, #39313, #39314, #39321, #39325, #39332, #39335, #39344, #39345, #39346, #39348, #39350, #39351
- Failed merges: