Make language around ToOwned for BorrowedFd more precise

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

@@ -24,9 +24,14 @@ use crate::sys_common::{AsInner, FromInner, IntoInner};
 /// 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.
+/// This type does not have a [`ToOwned`][crate::borrow::ToOwned]
+/// implementation. Calling `.to_owned()` on a variable of this type will call
+/// it on `&BorrowedFd` and use `Clone::clone()` like `ToOwned` does for all
+/// types implementing `Clone`. The result will be descriptor borrowed under
+/// the same lifetime.
+///
+/// To obtain an [`OwnedFd`], you can use [`BorrowedFd::try_clone_to_owned`]
+/// instead, but this is not supported on all platforms.
 #[derive(Copy, Clone)]
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(0)]
@@ -50,6 +55,8 @@ pub struct BorrowedFd<'fd> {
 /// descriptor, so it can be used in FFI in places where a file descriptor is
 /// passed as a consumed argument or returned as an owned value, and it never
 /// has the value `-1`.
+///
+/// You can use [`AsFd::as_fd`] to obtain a [`BorrowedFd`].
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(0)]
 // libstd/os/raw/mod.rs assures me that every libstd-supported platform has a