Rollup merge of #42252 - stjepang:clarify-alignof-docs, r=nikomatsakis
Rollup merge of #42252 - stjepang:clarify-alignof-docs, r=nikomatsakis

Clarify the docs for align_of and its variants

It's okay to have unaligned raw pointers and then use `ptr::write_unaligned` and `ptr::read_unaligned`.
However, using unaligned `&T` and `&mut T` would be undefined behavior.

The current documentation seems to indicate that everything has to be aligned, but in reality only references do. This PR changes the text of docs accordingly.

r? @sfackler

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