Rollup merge of #141770 - GuillaumeGomez:cfg-false-mod-rendering, r=camelid

Merge `Cfg::render_long_html` and `Cfg::render_long_plain` methods common code

Follow-up of rust-lang/rust#141747.

Thanks `@camelid` for spotting it!

r? `@camelid`

Author: matthiaskrgr