Rollup merge of #146136 - AudaciousAxiom:docs/missing-closing-code-block-fences, r=tgross35

docs(std): add missing closing code block fences in doc comments

This PR adds a few closing code block fences which I believe are missing in some doc comments. It seems that rustdoc just autocloses code blocks at the end of doc comments and thus these were easily overlooked: I do not think these code blocks are special in any way.

I found these when working on a Clippy lint that checks the last sentence of doc comments for terminal punctuation, and these were failing cases when testing against the std. Therefore I am not entirely sure these are all such cases, but still have high hopes that they are (or at least a well-defined subset of them).

Author: Zalathar