Comment flock usage on Linux

Author: nbdd0121

src/librustc_data_structures/flock.rs

@@ -12,6 +12,11 @@ use std::io;
 use std::path::Path;
 
 cfg_if! {
+    // We use `flock` rather than `fcntl` on Linux, because WSL1 does not support
+    // `fcntl`-style advisory locks properly (rust-lang/rust#72157).
+    //
+    // For other Unix targets we still use `fcntl` because it's more portable than
+    // `flock`.
     if #[cfg(target_os = "linux")] {
         use std::os::unix::prelude::*;