Updated Listing specific files to transfer (markdown)

Listing-specific-files-to-transfer.md

@@ -1,5 +1,7 @@
 You can create a list of specific files (or directories) to transfer, and then tell AzCopy to transfer only those exact files. As of version 10.3, this flag remains undocumented (except for this wiki page) and it does not appear in the in-app command line help. It is however regularly used by Azure Storage Explorer to pass file lists to its embedded copy of AzCopy.
 
+(The reason that it's undocumented is that, at high file counts, it has slow performance for downloads and service-to-service copies. We want to fix that before we document it more officially).
+
 To use this feature, create a text file that lists the files to be transferred, which are all under the root source directory.  
 
 Example file: