docs: add note about OS threads (#19225)

doc/docs.md

@@ -4059,6 +4059,13 @@ fn main() {
 }
 ```
 
+> **Note**
+> Threads rely on the machine's CPU (number of cores/threads).
+> Be aware that OS threads spawned with `spawn`
+> have limitations in regard to concurrency, 
+> including resource overhead and scalability issues, 
+> and might affect performance in cases of high thread count.
+
 There's also a `go` keyword. Right now `go foo()` will be automatically renamed via vfmt
 to `spawn foo()`, and there will be a way to launch a coroutine with `go` (a lightweight
 thread managed by the runtime).