Rollup merge of #41980 - gamazeps:thread-send, r=steveklabnik
Rollup merge of #41980 - gamazeps:thread-send, r=steveklabnik

[Doc] Add `'static` and `Send` constraints explanations to `thread::spawn`

Part of #29378.

Explains why the constraints on the closure and its return value are `'static` and `Send`.

Allows to tick of `thread::spawn` from the list of things to document in the `thread` module.

r? @steveklabnik

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