Clarify interface mismatch.

src/spawn.rs

@@ -342,8 +342,13 @@ impl Drop for PosixSpawnFileActions {
     }
 }
 
-// Specifically for use with posix_spawn and posix_spawnp.
-// https://github.com/rust-lang/libc/issues/1272
+// The POSIX standard requires those `args` and `envp` to be of type `*const *mut [c_char]`,
+// but implementations won't modify them, making the `mut` type redundant. Considering this,
+// Nix does not expose this mutability, but we have to change the interface when calling the
+// underlying libc interfaces , this helper function does the conversion job.
+//
+// SAFETY:
+// It is safe to add the mutability in types as implementations won't mutable them.
 unsafe fn to_exec_array<S: AsRef<CStr>>(args: &[S]) -> Vec<*mut libc::c_char> {
     let mut v: Vec<*mut libc::c_char> = args
         .iter()