Rollup merge of #25144 - killercup:docs/iter-fold-reduce, r=steveklabnik
Rollup merge of #25144 - killercup:docs/iter-fold-reduce, r=steveklabnik

Enhance Google-ability of `.fold()` by mentioning 'reduce' and 'inject' in the docs.

Motivation: [This thread on users.rust-lang.org](https://users.rust-lang.org/t/find-the-shortest-string-in-a-vector/1247)

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