Add maxStalenessSeconds to cursor.readPref

SERVER-27346 is addressing the absence of maxStalenessSeconds in the cursor.readPref shell method. Once the server ticket is closed, we need to:

• Uncomment the param in source/includes/apiargs-method-cursor.readPref-param.yaml
• Add maxStalenessSeconds to the method definition in source/reference/method/cursor.readPref.txt