Windows: Release Notes updates

Document ReparsePointPolicy registry value.

Document known Explorer Shell Paste bug.

Update ShortNames registry value.

Document Cygwin 1.7.18-1 support for AFS Symbolic Links.

Change-Id: Ifdcea9c71fb23d4e9cf9ad50907dd6f50c5c619e
Reviewed-on: http://gerrit.openafs.org/9869
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>