Update src/shims/unix/fd.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: tiif

src/shims/unix/fd.rs

@@ -641,8 +641,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         Ok(())
     }
 
-    /// This function either writes to the user supplied buffer and to dest place, or sets the
-    /// last libc error and writes -1 to dest.
+    /// Helper to implement `FileDescription::read`:
+    /// `result` should be the return value of some underlying `read` call that used `bytes` as its output buffer.
+    /// The length of `bytes` must not exceed either the host's or the target's `isize`.
+    /// If `Result` indicates success, `bytes` is written to `buf` and the size is written to `dest`.
+    /// Otherwise, `-1` is written to `dest` and the last libc error is set appropriately.
     fn return_read_bytes_and_count(
         &mut self,
         buf: Pointer,