Update src/shims/unix/fd.rs

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

Author: tiif

src/shims/unix/fd.rs

@@ -26,7 +26,9 @@ pub trait FileDescription: std::fmt::Debug + Any {
     fn name(&self) -> &'static str;
 
     /// Reads as much as possible into the given buffer, and returns the number of bytes read.
-    /// `ptr` is the pointer to user supplied read buffer.
+    /// `ptr` is the pointer to the user supplied read buffer.
+    /// `len` indicates how many bytes the user requested.
+    /// `dest` is where the return value should be stored.
     fn read<'tcx>(
         &self,
         _self_ref: &FileDescriptionRef,