Document that isize/usize are at least 16-bit wide on all platforms we currently support

[gnzlbg]

See this UCG PR: rust-lang/unsafe-code-guidelines#154

@briansmith raised the issue that this guarantee would be more useful if it was documented elsewhere and they are right. We should document that here, in the isize/usize API docs, and maybe also on the trait impls justified by this observation, like the From<u16> impl for usize, for example.

[Centril]

Sure, this seems reasonable to specify somewhere... but which section exactly... perhaps a new one "Assumptions about the platform"?

cc @ehuss

[gnzlbg]

Some trait impls are justified by this assumption, but we can't really attach documentation to trait impls AFAICT.

[Centril]

but we can't really attach documentation to trait impls AFAICT.

AFAIK we can actually.

[ehuss]

I would just include the minimum at https://doc.rust-lang.org/reference/types/numeric.html#machine-dependent-integer-types.

And: https://doc.rust-lang.org/reference/conditional-compilation.html#target_pointer_width