Add comment to std::os::unix::ffi::os_str explaining that the module is reused on other platforms.

Author: CDirkx

library/std/src/os/unix/ffi/os_str.rs

@@ -4,6 +4,9 @@ use crate::sealed::Sealed;
 use crate::sys::os_str::Buf;
 use crate::sys_common::{AsInner, FromInner, IntoInner};
 
+// Note: this file is currently reused in other `std::os::{platform}::ffi` modules to reduce duplication.
+// Keep this in mind when applying changes to this file that only apply to `unix`.
+
 /// Platform-specific extensions to [`OsString`].
 ///
 /// This trait is sealed: it cannot be implemented outside the standard library.