Extend core::char's documentation of casing issues

[Jules-Bertholet]

@rustbot label A-unicode A-docs

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@Manishearth would you have some time to review the added content here, in particular the reference to icu-casemap being the right crate to point at?

I'm also not sure about the extensive documentation of the specific cases - it seems OK, but it also feels like it probably ought to already be documented somewhere in Unicode and maybe we can just link out to it? That would make me feel more confident it stays up to date over time.

[Jules-Bertholet]

also feels like it probably ought to already be documented somewhere in Unicode and maybe we can just link out to it? That would make me feel more confident it stays up to date over time.

I wish lol. This information is widely dispersed between various Unicode data files, decade-old PDFs of proposals to the Unicode standards committee, Wayback Machine snapshots of defunct blogs, etc. I can link my sources if you would like, but as far as I am aware, there is no single authoritative prose resource we can direct users to for all this information.