Auto merge of #41814 - gamazeps:thread-struct-doc, r=steveklabnik

[Doc] improve `thread::Thread` and `thread::Builder` documentations

Part of #29378

- Adds information about the stack_size when using `Builder`. This might be considered too low level, but I assume that if someone wants to create their own builder instead of using `thread::spawn` they may be interested in that info.
- Updates the `thread::Thread` structure doc, mostly by explaining how to get one, the previous example was removed because it was not related to `thread::Thread`, but rather to `thread::Builder::name`.
  Not much is present there, mostly because this API is not often used (the only method that seems useful is `unpark`, which is documented in #41809).

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