improve windows tls documentation

Author: beepster4096

src/shims/tls.rs

@@ -244,10 +244,12 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             this.eval_windows("thread_local_key", "p_thread_callback")?.to_pointer(this)?;
         let thread_callback = this.get_ptr_fn(thread_callback)?.as_instance()?;
 
-        // Technically, the reason should be `DLL_PROCESS_DETACH` when the main thread exits but std ignores it.
+        // Technically, the reason should be `DLL_PROCESS_DETACH` when the main thread exits but std treats both the same.
         let reason = this.eval_windows("c", "DLL_THREAD_DETACH")?;
 
         // The signature of this function is `unsafe extern "system" fn(h: c::LPVOID, dwReason: c::DWORD, pv: c::LPVOID)`.
+        // `h` should be a handle to the current module and what `pv` should be is unknown
+        // but both are ignored by std
         this.call_function(
             thread_callback,
             Abi::System { unwind: false },