Add documentation about BorrowedFd::to_owned.

Following up on #88564, this adds documentation explaining why
`BorrowedFd::to_owned` returns another `BorrowedFd` rather than an
`OwnedFd`. And similar for `BorrowedHandle` and `BorrowedSocket`.

library/std/src/os/fd/owned.rs

@@ -21,6 +21,10 @@
 /// descriptor, so it can be used in FFI in places where a file descriptor is
 /// passed as an argument, it is not captured or consumed, and it never has the
 /// value `-1`.
+///
+/// This type's `.to_owned()` implementation returns another `BorrowedFd`
+/// rather than an `OwnedFd`. It just makes a trivial copy of the raw file
+/// descriptor, which is then borrowed under the same lifetime.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(0)]

library/std/src/os/windows/io/handle.rs

@@ -28,6 +28,10 @@
 /// And, it *may* have the value `NULL` (0), which can occur when consoles are
 /// detached from processes, or when `windows_subsystem` is used.
 ///
+/// This type's `.to_owned()` implementation returns another `BorrowedHandle`
+/// rather than an `OwnedHandle`. It just makes a trivial copy of the raw
+/// handle, which is then borrowed under the same lifetime.
+///
 /// [here]: https://devblogs.microsoft.com/oldnewthing/20040302-00/?p=40443
 #[derive(Copy, Clone)]
 #[repr(transparent)]

library/std/src/os/windows/io/socket.rs

@@ -21,6 +21,10 @@
 /// so it can be used in FFI in places where a socket is passed as an argument,
 /// it is not captured or consumed, and it never has the value
 /// `INVALID_SOCKET`.
+///
+/// This type's `.to_owned()` implementation returns another `BorrowedSocket`
+/// rather than an `OwnedSocket`. It just makes a trivial copy of the raw
+/// socket, which is then borrowed under the same lifetime.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(0)]