Optimize documentation lints **a lot** (1/2)

Turns out that `doc_markdown` uses a non-cheap rustdoc function
to convert from markdown ranges into source spans. And it was using it
a lot (about once every 18 lines of documentation on `tokio`, which ends
up being about 1800 times).

This ended up being about 18% of the total Clippy runtime as discovered by
lintcheck --perf in docs-heavy crates. This PR optimizes one of the cases
in which Clippy calls the function, and a future PR once
pulldown-cmark/pulldown-cmark issue number 1034 is merged will be open.

Note that not all crates were affected by this crate equally, those with
more docs are affected far more than those light ones.

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