Docs(lib): Fix `extract_if` docs
Various fixes to the documentation comments of the several `extract_if` collection methods available. It originally started with a small typo fix in `Vec`'s spotted when reading the 1.87 release notes, but then by looking at the others' for comparison in order to try determining what was the intended sentence, some inconsistencies were spotted. Therefore, some other changes are also proposed here to reduce these avoidable differences, going more and more nit-picky along the way. See the individual commits for more details about each change.
`@rustbot` label T-libs A-collections A-docs
3338ff7dcf