Clarify handling of `src` in `ptr::write`
Clarify handling of `src` in `ptr::write`

Fixes #39733.

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