Document that Option<char> has the same size and alignment as char on all platforms

[joshlf]

Filing to track https://github.com/rust-lang/reference/pull/1401/files#r1363998785

cc @joshtriplett

[madsmtm]

I'd suggest perhaps adding this to the docs for Option instead?

Or at least if it's added to the reference, then also document it on the primitive docs for char.