Replace `⇔` by `↔` in documentation (#52078)
There is no HTML entity `⇔` and accordingly all the usual browsers
(Chrome, Firefox, Safari) don't render it.
My guess is that there is confusion because GitHub supports `⇔` and
shows it as ⇔ (and `↔` as ↔).
But here is for example what Firefox shows:
<img width="853" alt="Screen Shot 2023-11-08 at 14 02 01"
src="https://github.com/JuliaLang/julia/assets/241512/af7d9e27-a5c3-46c3-94d9-82ed52653cb4">
This patch could be backported to 1.9 and 1.10.
