Address PR feedback

Add a few comments.

Author: stephentoub

src/System.IO.Compression/src/System/IO/Compression/DeflateStream.cs

@@ -265,6 +265,8 @@ public override int Read(byte[] array, int offset, int count)
                 }
                 else if (bytes > _buffer.Length)
                 {
+                    // The stream is either malicious or poorly implemented and returned a number of
+                    // bytes larger than the buffer supplied to it.
                     throw new InvalidDataException(SR.GenericInvalidData);
                 }
 
@@ -378,6 +380,8 @@ private async Task<int> ReadAsyncCore(Task<int> readTask, byte[] array, int offs
                     }
                     else if (bytesRead > _buffer.Length)
                     {
+                        // The stream is either malicious or poorly implemented and returned a number of
+                        // bytes larger than the buffer supplied to it.
                         throw new InvalidDataException(SR.GenericInvalidData);
                     }