Add comment explaining when posix_spawn() can be supported.

diff --git a/src/libstd/sys/unix/process/process_unix.rs b/src/libstd/sys/unix/process/process_unix.rs
index dcf0278b4aaa7c94481d4a195f0ac49fc130c11c..bd6a8d3f64bee895dff362e9c309d44def511c7a 100644 (file)
--- a/src/libstd/sys/unix/process/process_unix.rs
+++ b/src/libstd/sys/unix/process/process_unix.rs
@@ -242,6 +242,8 @@ fn posix_spawn(&mut self, _stdio: &ChildPipes, _envp: Option<&CStringArray>)
         Ok(None)
     }
 
+    // Only support platforms for which posix_spawn() can return ENOENT
+    // directly.
     #[cfg(any(target_os = "macos", target_os = "freebsd"))]
     fn posix_spawn(&mut self, stdio: &ChildPipes, envp: Option<&CStringArray>)
         -> io::Result<Option<Process>>