update handle documentation

Author: beepster4096

src/shims/windows/handle.rs

@@ -3,6 +3,9 @@ use std::mem::variant_count;
 
 use crate::*;
 
+/// A Windows `HANDLE` that represents a resource instead of being null or a pseudohandle.
+///
+/// This is a seperate type from [`Handle`] to simplify the packing and unpacking code.
 #[derive(Clone, Copy)]
 enum RealHandle {
     Thread(ThreadId),