Backup your Documents folder
You'll probably want to change c:\MyDocsBackup to a different location like \nas\Public\MyDocsBackup.
/r:1 means retry only once.
/w:1 means wait only one second for any open files to close before moving on.
/mir means mirror - so it will delete anything in the target folder that no longer exists in the source.
robocopy /r:1 /w:1 /mir %userprofile%\Documents c:\MyDocsBackup\Documents