Add comments about FileShare modes

Author: paulirwin

src/Lucene.Net/Store/MMapDirectory.cs

@@ -897,7 +897,9 @@ private static SharedMapping CreateAttempt(string file, int chunkSizePower)
                 // immediately recursively delete the directory. We need
                 // FileShare.Delete in particular: on Windows, a delete
                 // attempt against an open file fails unless the open
-                // share-mode permits FILE_SHARE_DELETE.
+                // share-mode permits FILE_SHARE_DELETE. In Java, FileChannel
+                // uses read+write+delete mode by default and Lucene doesn't
+                // override this.
                 //
                 // bufferSize: 1 because MemoryMappedFile uses only the
                 // file handle and bypasses the FileStream buffer, so a

src/Lucene.Net/Store/NIOFSDirectory.cs

@@ -104,6 +104,8 @@ public override IndexInput OpenInput(string name, IOContext context)
             EnsureOpen();
             EnsureCanRead(name); // LUCENENET-specific: backported call site from Lucene 6.0.0
             var path = Path.Combine(Directory.FullName, name); // LUCENENET specific: changed to use string file name instead of allocating a FileInfo (#832)
+            // LUCENENET NOTE: FileShare Read+Write+Delete is correct and matches Java Lucene.
+            // In Java, FileChannel defaults to this behavior, and Lucene does not override it.
             var fc = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete);
             return new NIOFSIndexInput("NIOFSIndexInput(path=\"" + path + "\")", fc, context);
         }
@@ -113,6 +115,8 @@ public override IndexInputSlicer CreateSlicer(string name, IOContext context)
             EnsureOpen();
             EnsureCanRead(name); // LUCENENET-specific: this method is not in Lucene 6.0.0 but added to match OpenInput above
             var path = Path.Combine(Directory.FullName, name); // LUCENENET specific: changed to use string file name instead of allocating a FileInfo (#832)
+            // LUCENENET NOTE: FileShare Read+Write+Delete is correct and matches Java Lucene.
+            // In Java, FileChannel defaults to this behavior, and Lucene does not override it.
             var fc = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete);
             return new IndexInputSlicerAnonymousClass(context, path, fc);
         }