Rollup merge of #77836 - RalfJung:transmute_copy, r=Mark-Simulacrum
Rollup merge of #77836 - RalfJung:transmute_copy, r=Mark-Simulacrum

transmute_copy: explain that alignment is handled correctly

The doc comment currently is somewhat misleading because if it actually transmuted `&T` to `&U`, a higher-aligned `U` would be problematic.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust