stty: Add documentation with respect to -F.

src/uu/stty/src/stty.rs

@@ -103,6 +103,15 @@ impl<'a> Options<'a> {
             all: matches.get_flag(options::ALL),
             save: matches.get_flag(options::SAVE),
             file: match matches.get_one::<String>(options::FILE) {
+                // Two notes here:
+                // 1. O_NONBLOCK is needed because according to GNU docs, a
+                //    POSIX tty can block waiting for carrier-detect if the
+                //    "clocal" flag is not set. If your TTY is not connected
+                //    to a modem, it is probably not relevant though.
+                // 2. We never close the FD that we open here, but the OS
+                //    will clean up the FD for us on exit, so it doesn't
+                //    matter. The alternative would be to have an enum of
+                //    BorrowedFd/OwnedFd to handle both cases.
                 Some(f) => std::fs::OpenOptions::new()
                     .read(true)
                     .custom_flags(libc::O_NONBLOCK)