Add docs comment for FileDescription::read's ptr

Author: tiif

src/shims/unix/fd.rs

@@ -26,6 +26,7 @@ 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.
     fn read<'tcx>(
         &self,
         _self_ref: &FileDescriptionRef,