Rollup merge of #126118 - jan-ferdinand:docs_for_vec_set_len, r=the8472

docs: Mention `spare_capacity_mut()` in `Vec::set_len`

I recently went down a small rabbit hole when trying to identify safe use of `Vec::set_len`. The solution was `Vec::spare_capacity_mut`. I think the docs on `Vec::set_len` benefit from mentioning this method.

A possible counter-argument could be that the [clippy lint `uninit_vec`](https://rust-lang.github.io/rust-clippy/master/index.html#/uninit_vec) already nudges people in the right direction. However, I think a working example on `Vec::set_len` is still beneficial.

Happy to hear your thoughts on the matter. :blush:

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