Remove incorrect docs from mem::transmute

[steveklabnik]

Fixes #22032

[rust-highfive]

r? @nikomatsakis

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[alexcrichton]

@bors: r+ e40d058 rollup

[reem]

Is there a reason for this not to be checked at compile time, or just implementation effort?

[steveklabnik]

@reem I asked @huonw if this was a bug or not, and he said that he hoped it wasn't checked, so maybe he can elaborate?

[huonw]

It doesn't seem to make sense to me for the alignment of the input value to matter for the output (the compiler just has to make sure the memory location storing the output has the right alignment), and it restricts things like transmuting SIMD types to/from arrays/tuples etc, and even say, (u8, u8) ↔ u16.