Clarify that RUST_MIN_STACK is internally cached

For larger applications it's important that users set `RUST_MIN_STACK`
at the start of their program because `min_stack` caches the value.
Not doing so can lead to their `env::set_var` call surprisingly not having any effect.

Author: seanlinsley

library/std/src/thread/mod.rs

@@ -131,7 +131,8 @@
 //!
 //! * Build the thread with [`Builder`] and pass the desired stack size to [`Builder::stack_size`].
 //! * Set the `RUST_MIN_STACK` environment variable to an integer representing the desired stack
-//!   size (in bytes). Note that setting [`Builder::stack_size`] will override this.
+//!   size (in bytes). Note that setting [`Builder::stack_size`] will override this. Also, note
+//!   that `RUST_MIN_STACK` is cached internally so should be set before any threads are started.
 //!
 //! Note that the stack size of the main thread is *not* determined by Rust.
 //!