Expand std::os::unix::fs::chown() doc with a warning

Include warning about losing setuid/gid when chowning, per POSIX.

std/src/os/unix/fs.rs

@@ -986,6 +986,10 @@ impl DirBuilderExt for fs::DirBuilder {
 /// Changing the owner typically requires privileges, such as root or a specific capability.
 /// Changing the group typically requires either being the owner and a member of the group, or
 /// having privileges.
+/// 
+/// Be aware that changing owner clears the `suid` and `sgid` permission bits in most cases
+/// according to POSIX, usually even if the user is root. The sgid is not cleared when
+/// the file is non-group-executable.
 ///
 /// If called on a symbolic link, this will change the owner and group of the link target. To
 /// change the owner and group of the link itself, see [`lchown`].