Rollup merge of #141747 - lolbinarycat:rustdoc-cfg-138112, r=GuillaumeGomez

rustdoc: display doc(cfg(false)) properly

before we had an extra 'on' that was
ungramatical.

fixes rust-lang/rust#138112

this is what it looks like now:
![screenshot: Available nowhere](https://github.com/user-attachments/assets/e27b4990-09a7-4f13-8bcf-26d44c8c1bea)

Author: matthiaskrgr