[Issue 19348] Struct casts should be better documented.

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19348

--- Comment #3 from Nick Treleaven <nick at geany.org> ---
> No constructor at all seems to be called

Sorry, that was wrong. Casting tries S(v) first, then for casting from struct
or static array, it falls back to reinterpret casts. I'm making a PR to
document this.

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