Add docs for sys/file.h

src/header/sys_file/mod.rs

@@ -1,4 +1,6 @@
-//! sys/file.h implementation
+//! `sys/file.h` implementation.
+//!
+//! Non-POSIX, see <https://man7.org/linux/man-pages/man2/flock.2.html>.
 
 use crate::{
     error::ResultExt,
@@ -14,6 +16,7 @@ pub const L_SET: usize = 0;
 pub const L_INCR: usize = 1;
 pub const L_XTND: usize = 2;
 
+/// See <https://man7.org/linux/man-pages/man2/flock.2.html>.
 #[no_mangle]
 pub extern "C" fn flock(fd: c_int, operation: c_int) -> c_int {
     Sys::flock(fd, operation).map(|()| 0).or_minus_one_errno()