Rollup merge of #24440 - nham:improve_enumerate_doc, r=alexcrichton
Rollup merge of #24440 - nham:improve_enumerate_doc, r=alexcrichton

I found that the current description of `enumerate()` doesn't actually tell you what, specifically, the method does, and you have to look at the example to figure it out. Here's a description that I think is better.

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