Add #[must_use] to thread::Builder

[jkugelman]

I copied the wording of the fmt::Debug builders.

Affects:

std/src/thread/mod.rs:289:5   std::thread::Builder   fn new() -> Builder;
std/src/thread/mod.rs:318:5   std::thread::Builder   fn name(mut self, name: String) -> Builder;
std/src/thread/mod.rs:341:5   std::thread::Builder   fn stack_size(mut self, size: usize) -> Builder;

Parent issue: #89692

r? @joshtriplett

[joshtriplett]

There's more than one spawn function, so I made the message more generic.

[joshtriplett]

@bors r+

[bors]

📌 Commit 6a8311c has been approved by joshtriplett