Document semantics of add_entry parameter to readdir callback.

Fixes: #196.

src/api/filesystem/async_io.rs

@@ -618,7 +618,9 @@ pub trait AsyncFileSystem: FileSystem {
         /// the file system did not return a `Handle` from `opendir` then the contents of `handle` are
         /// undefined.
         ///
-        /// `size` indicates the maximum number of bytes that should be returned by this method.
+        /// `size` indicates the maximum number of bytes that should be returned by this method,
+        /// but can be ignored in practice. Instead, entries are returned by calling the
+        /// `add_entry` function until it returns zero (indicating that the buffer is full).
         ///
         /// If `offset` is non-zero then it corresponds to one of the `offset` values from a `DirEntry`
         /// that was previously returned by a call to `readdir` for the same handle. In this case the

src/api/filesystem/sync_io.rs

@@ -634,7 +634,10 @@ pub trait FileSystem {
     /// the file system did not return a `Handle` from `opendir` then the contents of `handle` are
     /// undefined.
     ///
-    /// `size` indicates the maximum number of bytes that should be returned by this method.
+    /// `size` indicates the maximum number of bytes that should be returned by this method,
+    /// but can be ignored in practice. Instead, entries are returned by calling the
+    /// `add_entry` function until it returns zero (indicating that the buffer is full).
+    ///
     ///
     /// If `offset` is non-zero then it corresponds to one of the `offset` values from a `DirEntry`
     /// that was previously returned by a call to `readdir` for the same handle. In this case the