Rollup merge of #87848 - godmar:@godmar/thread-join-documentation-fix, r=joshtriplett
Rollup merge of #87848 - godmar:@godmar/thread-join-documentation-fix, r=joshtriplett

removed references to parent/child from std::thread documentation

- also clarifies how thread.join and detaching of threads works
- the previous prose implied that there is a relationship between a
spawning thread and the thread being spawned, and that "child" threads
couldn't outlive their "parents" unless detached, which is incorrect.

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