Rewrite docs for stack size/thread names for spawned threads.
Rewrite docs for stack size/thread names for spawned threads.

* Moves docs about stack size and thread naming from `Builder` to the
  `std::thread` module
* Adds more links to the new module-level documentation
* Mentions the 2 MiB stack size default, but indicate it's subject to
  change

Fixes https://github.com/rust-lang/rust/issues/43805.

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