Rollup merge of #51152 - crlf0710:patch-1, r=kennytm
Rollup merge of #51152 - crlf0710:patch-1, r=kennytm

Replace `if` with `if and only if` in the definition dox of `Sync`

The old text was: "The precise definition is: a type `T` is `Sync` if `&T` is Send."

Since we've also got
```
impl<'a, T> Send for &'a T
where
    T: Sync + ?Sized,
```
I purpose we can change the `if` to `if and only if` to make it more precise.

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