Add context to PopulatedDeleteFileIndex (#1084)

## What changes are included in this PR?
Added some doc for `PopulatedDeleteFileIndex`

---------

Co-authored-by: Renjie Liu <[email protected]>

Author: jonathanc-n

crates/iceberg/src/delete_file_index.rs

@@ -94,6 +94,14 @@ impl DeleteFileIndex {
 }
 
 impl PopulatedDeleteFileIndex {
+    /// Creates a new populated delete file index from a list of delete file contexts, which
+    /// allows for fast lookup when determining which delete files apply to a given data file.
+    ///
+    /// 1. The partition information is extracted from each delete file's manifest entry.
+    /// 2. If the partition is empty and the delete file is not a positional delete,
+    ///    it is added to the `global_delees` vector
+    /// 3. Otherwise, the delete file is added to one of two hash maps based on its content type.
+
     fn new(files: Vec<DeleteFileContext>) -> PopulatedDeleteFileIndex {
         let mut eq_deletes_by_partition: HashMap<Struct, Vec<Arc<DeleteFileContext>>> =
             HashMap::default();