Auto merge of #142910 - yotamofek:pr/rustdoc/markdown-lazy-to-string, r=GuillaumeGomez

Lazy-ify some markdown rendering

Seems to have a positive effect in my local perf runs 😍

r? `@GuillaumeGomez` if you're interested, otherwise feel free to reassign
(would also love a perf run)

Author: bors