Rollup merge of #41994 - gamazeps:thread-builder, r=GuillaumeGomez

[Doc] Implrove `thread::Builder`'s doc.

Part of #29378 .

- Explains *why* we would use the builder instead ofthe free function.
- Changes the parent-child explanation for a spawned-caller in `thread::Builder::spawn`
- Adds a link to `io::Result` in `thread::Builder`
- Corrects the return type doc in `thread::Builder::spawn`

r? @rust-lang/docs

diff --cc src/libstd/thread/mod.rs
Simple merge