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

[gamazeps]

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

[nagisa]

Thread being able to outlive the scope it has been created in (i.e. detaching) is only an explanation for why closure itself is 'static, and not its return value. The return value has the 'static bound because the return value outlives the thread it is returned from; this is true regardless of whether the thread gets detached or not.

[gamazeps]

They are actually linked.

The value a thread returns must outlive the closure, but the fact that the thread can detach implies that the closure must be 'static, since the value must live at least as long as the closure it must then also be 'static

[gamazeps]

r? @rust-lang/docs

[GuillaumeGomez]

Tidy errors:

[00:01:44] tidy error: /checkout/src/libstd/thread/mod.rs:376: trailing whitespace
[00:01:44] tidy error: /checkout/src/libstd/thread/mod.rs:385: trailing whitespace

[gamazeps]

Damn, that is valid markdown though...
Fixed

[Mark-Simulacrum]

@steveklabnik This looks read for review.

[steveklabnik]

@bors: r+ rollup

thanks!

[bors]

📌 Commit 770bd57 has been approved by steveklabnik