"Function to check whether a non-file buffer needs reverting.
This should be a function with one optional argument NOCONFIRM.
Auto Revert Mode sets NOCONFIRM to t. The function should return
-non-nil if the buffer should be reverted. The buffer is current
-when this function is called.
+non-nil if the buffer should be reverted. A return value of
+`fast' means that the need for reverting was not checked, but
+that reverting the buffer is fast. The buffer is current when
+this function is called.
The idea behind the NOCONFIRM argument is that it should be
non-nil if the buffer is going to be reverted without asking the