Document that Err(io::Errno::INVAL) is returned when FlockType is…

… unlocked in `fcntl_getlk`

src/backend/libc/process/syscalls.rs

@@ -666,6 +666,8 @@ pub(crate) fn getgroups(buf: &mut [Gid]) -> io::Result<usize> {
 )))]
 #[inline]
 pub(crate) fn fcntl_getlk(fd: BorrowedFd<'_>, lock: &Flock) -> io::Result<Option<Flock>> {
+    // In order to guarantee consistent behavior across all POSIX platforms, return `EINVAL` when
+    // `flock.l_type` is `F_UNLCK`
     if lock.typ == crate::process::FlockType::Unlocked {
         return Err(io::Errno::INVAL);
     }

src/process/fcntl_getlk.rs

@@ -2,7 +2,10 @@ use super::Flock;
 use crate::fd::AsFd;
 use crate::{backend, io};
 
-/// `fcntl(fd, F_GETLK)`—Get the first lock that blocks the lock description pointed to by the argument `lock`. If no such lock is found, then `None` is returned
+/// `fcntl(fd, F_GETLK)`—Get the first lock that blocks the lock description pointed to by the
+/// argument `lock`. If no such lock is found, then `None` is returned
+///
+/// If `lock.typ` is `FlockType::Unlocked`, then `Err(io::Errno::INVAL)` is returned
 ///
 /// # References
 ///  - [POSIX]