doc: be more clear about what the 'spawn' does

(cherry picked from commit 2e071c2)

Author: bishopcheckmate

rayon-core/src/spawn/mod.rs

@@ -5,8 +5,8 @@ use crate::unwind;
 use std::mem;
 use std::sync::Arc;
 
-/// Fires off a task into the Rayon threadpool in the "static" or
-/// "global" scope.  Just like a standard thread, this task is not
+/// Puts the task into the Rayon threadpool's job queue in the "static"
+/// or "global" scope. Just like a standard thread, this task is not
 /// tied to the current stack frame, and hence it cannot hold any
 /// references other than those with `'static` lifetime. If you want
 /// to spawn a task that references stack data, use [the `scope()`